bcachefs: Fix stripe_idx_to_delete()

There was a null ptr deref when there wasn't a stripes heap allocated
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/ec.c

@@ -575,7 +575,8 @@ static ssize_t stripe_idx_to_delete(struct bch_fs *c)
 {
 	ec_stripes_heap *h = &c->ec_stripes_heap;
 
-	return h->data[0].blocks_nonempty == 0 ? h->data[0].idx : -1;
+	return h->used && h->data[0].blocks_nonempty == 0
+		? h->data[0].idx : -1;
 }
 
 static inline int ec_stripes_heap_cmp(ec_stripes_heap *h,

fs/bcachefs/super.c

@@ -277,6 +277,7 @@ void bch2_fs_read_only(struct bch_fs *c)
 	 */
 	percpu_ref_kill(&c->writes);
 
+	cancel_work_sync(&c->ec_stripe_delete_work);
 	cancel_delayed_work(&c->pd_controllers_update);
 
 	/*