bcachefs: Fix assertion pop in bch2_ptr_swab()

This runs on extents that haven't yet been validated, so we don't want
to assert that we have a valid entry type.

Reported-by:  <syzbot+4f29c3f12f864d8a8d17@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>