Fix a potential memory leak in the NFS over TLS handling code.

For the TLS case where there is a "user@domain" name specified in the
X.509 v3 certificate presented by the client in the otherName component
of subjectAltName, a gid list is allocated via mem_alloc().
This needs to be free'd. Otherwise xp_gidp == NULL and free() handles that.
(The size argument to mem_free() is not used by FreeBSD, so it can be 0.)

This leak would not have occurred for any other case than NFS over TLS
with the "user@domain" in the client's certificate.