Don't cleanup uninitialized dns_resolver buckets

Don't cleanup uninitialized dns_resolver buckets

If the isc_task_create_bound() fails in the middle of buckets
initialization - the most common case would be shutdown initialized
during reload, not all tasks would be initialized, but the cleanup
code would try to cleanup all buckets.

Make sure that we cleanup only the initialized buckets by setting
ntasks to the number of already initialized tasks on the error path.