Solution: remove it.
Looking at the code:

https://github.com/krb5/krb5/blob/master/src/lib/gssapi/mechglue/g_unseal.c#L55

gss_unwrap as the very first thing checks that plaintext is not a
null pointer, which in our case it's true given it's on the stack,
and then initialises its members to 0 length and null ptr.

https://github.com/krb5/krb5/blob/master/src/lib/gssapi/mechglue/g_rel_buffer.c#L36

So it should be safe to release it in all cases, and the release API
seems to check again if it's not a null pointer and then if the
members are 0 length and null pointer it's a no-op.