fsck: do not dereference NULL while checking resolve-undo data

When we found an invalid object recorded in the resolve-undo data,
we would have ended up dereferencing NULL while fsck.  Reporting the
problem and going on to the next object is the right thing to do
here.

Noticed by SZEDER Gábor.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fsck.c

@@ -784,6 +784,7 @@ static int fsck_resolve_undo(struct index_state *istate)
 				error(_("%s: invalid sha1 pointer in resolve-undo"),
 				      oid_to_hex(&ru->oid[i]));
 				errors_found |= ERROR_REFS;
+				continue;
 			}
 			obj->flags |= USED;
 			fsck_put_object_name(&fsck_walk_options, &ru->oid[i],