PGLog: fix clear() to avoid the IndexLog::zero() asserts

Introduced in:
  c5b8d8105d965da852c79add607b69d5ae79a4d4
  ac11ca40b4f4525cbe9b1778b1c5d9472ecb9efa
Signed-off-by: Samuel Just <sam.just@inktank.com>