Fix a crash in ls-remote when refspec expands into nothing

Originally-by: Väinö Järvelä <v@pp.inet.fi>
Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

remote.c

@@ -909,7 +909,8 @@ int get_fetch_map(struct ref *remote_refs,
 			    rm->peer_ref->name);
 	}
 
-	tail_link_ref(ref_map, tail);
+	if (ref_map)
+		tail_link_ref(ref_map, tail);
 
 	return 0;
 }