The invocation of va_start() is missing a corresponding va_end in function 'diag(const char *fmt, ...)' when a return is called. This is already fixed in Upstream[1] and it is a very simple one-liner.

[1]https://github.com/zorgnax/libtap/blob/b53e4ef5257f80e881762b6143834d8aae29da1a/tap.c#L233