bcachefs: Fix a rare path in bch2_btree_path_peek_slot()

bcachefs: Fix a rare path in bch2_btree_path_peek_slot()

In the drop_alloc tests, we may end up calling
bch2_btree_iter_peek_slot() on an interior level that doesn't exist.
Previously, this would hit the path->uptodate assertion in
bch2_btree_path_peek_slot(); this path first checks a NULL btree node,
which is how we know we're at the end of the btree.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>