MB-10137 Initialize max_purged_seq field of compaction_ctx to zero

There exists an ambiguity about zero initialization of builtin
types for data members of POD types. Added an explicit statement
to set max_purged_seq to zero.

Change-Id: I5be9d158fd08c72715326e8dc454497ba83cae7f
Reviewed-on: http://review.couchbase.org/33785
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: Ketaki Gangal <ketakigangal@gmail.com>