fetch: do not create ref from empty name

Previously, the refspec "<src>:" would be expanded to
"<src>:refs/heads/". Instead, treat an empty <dst> just like refspecs
without a colon.
Signed-off-by: NClemens Buchacher <drizzd@aon.at>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote.c

@@ -1254,7 +1254,7 @@ struct ref *get_remote_ref(const struct ref *remote_refs, const char *name)
 
 static struct ref *get_local_ref(const char *name)
 {
-	if (!name)
+	if (!name || name[0] == '\0')
 		return NULL;
 
 	if (!prefixcmp(name, "refs/"))