Description of problem:

Oc-mirror should not panic when failed to get  release signature

Version-Release number of selected component (if applicable):

oc-mirror version WARNING: This version information is deprecated and will be replaced with the output from --short. Use --output=yaml|json to get the full version.Client Version: version.Info{Major:"", Minor:"", GitVersion:"4.17.0-202407291514.p0.gdbf115f.assembly.stream.el9-dbf115f", GitCommit:"dbf115f547a19f12ab72e7b326be219a47d460a0", GitTreeState:"clean", BuildDate:"2024-07-29T15:52:52Z", GoVersion:"go1.22.4 (Red Hat 1.22.4-2.el9) X:strictfipsruntime", Compiler:"gc", Platform:"linux/amd64"}

How reproducible:

100%

Steps to Reproduce:

1)  Mirror2disk+disk2mirror with following imagesetconfig, and mirror to enterprise registry :
kind: ImageSetConfiguration
apiVersion: mirror.openshift.io/v2alpha1
mirror:
  platform:
    channels:
    - name: stable-4.15                                             
      type: ocp
      minVersion: '4.15.18'
      maxVersion: '4.15.18'

2) Setup squid and only with white list with enterprise registry and the OSUS service ;
cat /etc/squid/squid.conf
http_port 3128
coredump_dir /var/spool/squid
acl whitelist dstdomain "/etc/squid/whitelist"
http_access allow whitelist
http_access deny !whitelist

cat /etc/squid/whitelist 
my-route-zhouy.apps.yinzhou-88.qe.devcluster.openshift.com             -------------registry route  (oc get route -n your registry app's project)
update-service-oc-mirror-route-openshift-update-service.apps.yinzhou-88.qe.devcluster.openshift.com        ---osus route  (oc get route -n openshift-update-service)

Sudo systemctl restart squid
export https_proxy=http://127.0.0.1:3128
export http_proxy=http://127.0.0.1:3128

3)  Setting registry redirect with : 
cat ~/.config/containers/registries.conf 
[[registry]]
  location = "quay.io"
  insecure = false
  blocked = false
  mirror-by-digest-only = false
  prefix = ""
  [[registry.mirror]]
    location = "my-route-zhouy.apps.yinzhou-88.qe.devcluster.openshift.com"
    insecure = false


4) Use the same imagesetconfig and mirror to a new folder :
`oc-mirror -c config-38037.yaml file://new-folder --v2`

Actual results:

4) the oc-mirror command panic with error :  

I0812 06:45:26.026441  199941 core-cincinnati.go:508] Using proxy 127.0.0.1:3128 to request updates from https://update-service-oc-mirror-route-openshift-update-service.apps.yinzhou-417.qe.devcluster.openshift.com/api/upgrades_info/v1/graph?arch=amd64&channel=stable-4.15&id=a6097264-8b29-438f-9e71-4aba1e9ec32d
2024/08/12 06:45:26  [ERROR]  : http request Get "https://mirror.openshift.com/pub/openshift-v4/signatures/openshift/release/sha256=0f55261077557d1bb909c06b115e0c79b0025677be57ba2f045495c11e2443ee/signature-1": Forbidden
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x3d1e3f6]
 
goroutine 1 [running]:
github.com/openshift/oc-mirror/v2/internal/pkg/release.SignatureSchema.GenerateReleaseSignatures({

, {0x4c7b348, 0x15}, {0xc000058c60, 0x1c, {...}, {...}, {...}, {...}, ...}, ..., ...}, ...)
/home/fedora/yinzhou/oc-mirror/vendor/github.com/openshift/oc-mirror/v2/internal/pkg/release/signature.go:97 +0x676
github.com/openshift/oc-mirror/v2/internal/pkg/release.(*CincinnatiSchema).GetReleaseReferenceImages(0xc0007203c0, {0x55caf68, 0x764c060})
/home/fedora/yinzhou/oc-mirror/vendor/github.com/openshift/oc-mirror/v2/internal/pkg/release/cincinnati.go:230 +0x70b
github.com/openshift/oc-mirror/v2/internal/pkg/release.(*LocalStorageCollector).ReleaseImageCollector(0xc000b12388, {0x55caf68, 0x764c060})
/home/fedora/yinzhou/oc-mirror/vendor/github.com/openshift/oc-mirror/v2/internal/pkg/release/local_stored_collector.go:58 +0x407
github.com/openshift/oc-mirror/v2/internal/pkg/cli.(*ExecutorSchema).CollectAll(0xc000ae8908, {0x55caf68, 0x764c060})
/home/fedora/yinzhou/oc-mirror/vendor/github.com/openshift/oc-mirror/v2/internal/pkg/cli/executor.go:955 +0x122
github.com/openshift/oc-mirror/v2/internal/pkg/cli.(*ExecutorSchema).RunMirrorToDisk(0xc000ae8908, 0xc0005f3b08, {0xa?, 0x20?, 0x20?})
/home/fedora/yinzhou/oc-mirror/vendor/github.com/openshift/oc-mirror/v2/internal/pkg/cli/executor.go:707 +0x1aa
github.com/openshift/oc-mirror/v2/internal/pkg/cli.(*ExecutorSchema).Run(0xc000ae8908, 0xc0005f1640?, {0xc0005f1640?, 0x0?, 0x0?})
/home/fedora/yinzhou/oc-mirror/vendor/github.com/openshift/oc-mirror/v2/internal/pkg/cli/executor.go:459 +0x149
github.com/openshift/oc-mirror/v2/internal/pkg/cli.NewMirrorCmd.func1(0xc0005f3b08, {0xc0005f1640, 0x1, 0x4})
/home/fedora/yinzhou/oc-mirror/vendor/github.com/openshift/oc-mirror/v2/internal/pkg/cli/executor.go:207 +0x32a
github.com/spf13/cobra.(*Command).execute(0xc0005f3b08, {0xc000166010, 0x4, 0x4})
/home/fedora/yinzhou/oc-mirror/vendor/github.com/spf13/cobra/command.go:987 +0xab1
github.com/spf13/cobra.(*Command).ExecuteC(0xc0005f3b08)
/home/fedora/yinzhou/oc-mirror/vendor/github.com/spf13/cobra/command.go:1115 +0x3ff
github.com/spf13/cobra.(*Command).Execute(0x741ec38?)
/home/fedora/yinzhou/oc-mirror/vendor/github.com/spf13/cobra/command.go:1039 +0x13
main.main()
/home/fedora/yinzhou/oc-mirror/cmd/oc-mirror/main.go:10 +0x18
 

Expected results:

The command could fail, but not panic