mon/OSDMonitor: fix bug in try_prune_purged_snaps

mon/OSDMonitor: fix bug in try_prune_purged_snaps

If 'begin' isn't found, we'll get a [pbegin,pend) range back that was
nearby.  Only if it overlaps the [begin,end) range do we want to shorten
our range to [begin,pbegin); the old assert was making the assumption
that the lookup would only return a range that was after 'begin', but in
reality it can return was that comes before it too.

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