Ensure that invalid mem tracking hooks return false

The 'invalid' memory tracking hooks are used when memcached is not
linked to tcmalloc. Unfortuneately they return -1 which is incorrectly
interpreteted as success in mc_{add,remove}_{new,delete}_hook()
functions:

    bool mc_add_new_hook(void (*hook)(const void* ptr, size_t size)) {
        return addNewHook(hook) ? true : false;
    }

Change them to return zero on failure, as per the TCMalloc function
signatures.

Change-Id: Id4edc598e04bfa0b7680fe8ee278847fea3e27fa
Reviewed-on: http://review.couchbase.org/42464
Reviewed-by: Daniel Owen <owend@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>