fix segfault showing an empty remote

In case of an empty list, the search for its tail caused a
NULL-pointer dereference.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Reported-by: Erik Faye-Lund <kusmabite@googlemail.com>
Acked-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-remote.c

@@ -299,11 +299,11 @@ static int get_push_ref_states(const struct ref *remote_refs,
 		return 0;
 
 	local_refs = get_local_heads();
-	ref = push_map = copy_ref_list(remote_refs);
-	while (ref->next)
-		ref = ref->next;
-	push_tail = &ref->next;
+	push_map = copy_ref_list(remote_refs);
 
+	push_tail = &push_map;
+	while (*push_tail)
+		push_tail = &((*push_tail)->next);
 	match_refs(local_refs, push_map, &push_tail, remote->push_refspec_nr,
 		   remote->push_refspec, MATCH_REFS_NONE);
 

t/t5505-remote.sh

@@ -494,5 +494,15 @@ test_expect_success 'remote prune to cause a dangling symref' '
 	grep "dangling symref" err
 '
 
+test_expect_success 'show empty remote' '
+
+	test_create_repo empty &&
+	git clone empty empty-clone &&
+	(
+		cd empty-clone &&
+		git remote show origin
+	)
+'
+
 test_done