bcachefs: backpointer_get_key: check for null from peek_slot()

peek_slot() doesn't normally return bkey_s_c_null - except when we ask
for a key at a btree level that doesn't exist, which can happen here.

We might want to revisit this, but we'll have to look over all the
places where we use peek_slot() on interior nodes.

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