Ephemeral port number (port="0") should not be affected by port offset

Port 0 is almost universally considered a wildcard port/indeterminate port/ephemeral/port chosen by the OS and obviously applying an offset to such port (e.g. offset of -100) will fail on multiple fronts, e.g.:

Caused by: java.lang.IllegalArgumentException: port out of range:-100

This is a regression introduced in https://issues.jboss.org/browse/WFLY-2345 based on discussion on https://developer.jboss.org/message/842406#842407 in PR https://github.com/wildfly/wildfly/pull/5307/files#diff-1a9ef995f085779b0598a1322551e580L114

The original condition was also not really correct, since we allow negative and positive port offsets, the correct condition really is

if (!isFixedPort && port != 0) {