bcachefs: Fix null ptr deref in invalidate_one_bucket()

bch2_backpointer_get_key() returns bkey_s_c_null when the target isn't
found.

backpointer_get_key() flags the error, so there's nothing else to do
here - just skip it and move on.

Link: koverstreet/bcachefs#847
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/alloc_background.c

@@ -2084,6 +2084,9 @@ static int invalidate_one_bp(struct btree_trans *trans,
 	if (ret)
 		return ret;
 
+	if (!extent_k.k)
+		return 0;
+
 	struct bkey_i *n =
 		bch2_bkey_make_mut(trans, &extent_iter, &extent_k,
 				   BTREE_UPDATE_internal_snapshot_node);