MB-20586: darwin_zone: Use correct handle for zone_calloc
MB-20586: darwin_zone: Use correct handle for zone_calloc

Fix a typo in our darwin zone allocator / shim - use the underlying
zone handle when calling into default_zone->calloc().

This appears to be a benign issue when the default zone is jemalloc
(as is the standard deployment), but can causes segfaults when using
the system zone allocator.

Change-Id: I4d525a5482766b5a6f5b272cf6f994d00dc9cc35
Reviewed-on: http://review.couchbase.org/67086
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Well-Formed: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: David Haikney <429c6ca43f70a1a1104bd68d6b2835bd23bff953@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

daverigby/kv_engine,daverigby/memcached,daverigby/kv_engine,couchbase/memcached,couchbase/memcached,daverigby/memcached,daverigby/memcached,daverigby/kv_engine,couchbase/memcached,daverigby/kv_engine,daverigby/memcached,couchbase/memcached