builtin-fetch: Don't segfault on "fetch +foo"

If we are fetching something and were configured to do a forced
fetch and have no local ref to store the fetched object into we
cannot mark the local ref as having a forced update.  Instead we
should just silently discard the + request.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

remote.c

@@ -875,8 +875,7 @@ int get_fetch_map(struct ref *remote_refs,
 					 refspec->src : "HEAD");
 
 		ref_map->peer_ref = get_local_ref(refspec->dst);
-
-		if (refspec->force)
+		if (ref_map->peer_ref && refspec->force)
 			ref_map->peer_ref->force = 1;
 	}