push: fix segfault for odd config

If you have a branch.$X.merge config option, but no branch.$X.remote, and
your configuration tries to push tracking branches, git will segfault.

The problem is that even though branch->merge_nr is 1, you don't actually
have an upstream since there is no remote.  Other callsites generally
check explicitly that branch->merge is not NULL, so let's do that here,
too.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-push.c

@@ -54,7 +54,7 @@ static void setup_push_tracking(void)
 	struct branch *branch = branch_get(NULL);
 	if (!branch)
 		die("You are not currently on a branch.");
-	if (!branch->merge_nr)
+	if (!branch->merge_nr || !branch->merge)
 		die("The current branch %s is not tracking anything.",
 		    branch->name);
 	if (branch->merge_nr != 1)