osd/PG: peek_map_epoch: skip legacy PGs if infos object is missing

osd/PG: peek_map_epoch: skip legacy PGs if infos object is missing

- pg is removed
- osd is stopped before pg is fully removed
- on restart, we ignore/skip the pg because its epoch is too old
- we upgrade to hammer and convert other pgs, skipping this one, and then
  remove the legacy infos object
- hammer starts, tries to parse the legacy pg, and fails because the infos
  object is gone, crashing.

The fix is to continue ignoring the zombie pg.

Fixes: #16030
Signed-off-by: Sage Weil <sage@redhat.com>