MB-15780: Fix use of non-NUL terminated string in JSON topkeys
MB-15780: Fix use of non-NUL terminated string in JSON topkeys

When forming the JSON topkeys structure, the key was incorrectly
assumed to be NUL-terminated.

Change-Id: I0e3f90d0f255a338e5dd7a33c195548864e3078d
Reviewed-on: http://review.couchbase.org/53606
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

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