push: fix segfault when HEAD points nowhere

After a push of a branch other than the current branch fails in
a no-ff error and if you are still on an unborn branch, the code
recently added to report the failure dereferenced a null pointer
while checking the name of the current branch.

Signed-off-by: Fraser Tweedale <frase@frase.id.au>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport.c

@@ -744,7 +744,7 @@ void transport_print_push_status(const char *dest, struct ref *refs,
 			n += print_one_push_status(ref, dest, n, porcelain);
 		if (ref->status == REF_STATUS_REJECT_NONFASTFORWARD &&
 		    *nonfastforward != NON_FF_HEAD) {
-			if (!strcmp(head, ref->name))
+			if (head != NULL && !strcmp(head, ref->name))
 				*nonfastforward = NON_FF_HEAD;
 			else
 				*nonfastforward = NON_FF_OTHER;