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.

sys/rpc/svc.c

@@ -902,6 +902,8 @@ svc_xprt_free(SVCXPRT *xprt)
 {
 
 	mem_free(xprt->xp_p3, sizeof(SVCXPRT_EXT));
+	/* The size argument is ignored, so 0 is ok. */
+	mem_free(xprt->xp_gidp, 0);
 	mem_free(xprt, sizeof(SVCXPRT));
 }