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: https://github.com/koverstreet/bcachefs/issues/847


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