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>

upload-pack.c

@@ -226,7 +226,7 @@ static int send_ref(const char *refname, const unsigned char *sha1)
 		nr_our_refs++;
 	}
 	if (o->type == tag_type) {
-		o = deref_tag(o);
+		o = deref_tag(o, refname, 0);
 		packet_write(1, "%s %s^{}\n", sha1_to_hex(o->sha1), refname);
 	}
 	return 0;