Be careful when dereferencing tags.

One caller of deref_tag() was not careful enough to make sure
what deref_tag() returned was not NULL (i.e. we found a tag
object that points at an object we do not have).  Fix it, and
warn about refs that point at such an incomplete tag where
needed.

Signed-off-by: Junio C Hamano <junkio@cox.net>

tag.c

@@ -3,10 +3,15 @@
 
 const char *tag_type = "tag";
 
-struct object *deref_tag(struct object *o)
+struct object *deref_tag(struct object *o, const char *warn, int warnlen)
 {
 	while (o && o->type == tag_type)
 		o = parse_object(((struct tag *)o)->tagged->sha1);
+	if (!o && warn) {
+		if (!warnlen)
+			warnlen = strlen(warn);
+		error("missing object referenced by '%.*s'", warnlen, warn);
+	}
 	return o;
 }