Resolve violation of weak referencing dns_view

The dns_view implements weak and strong reference counting. When strong reference counting reaches zero, the adb, ntatable and resolver objects are shut down and detached.

In dns_zone and dns_nta the dns_view was weakly attached, but the view->resolver reference was accessed directly leading to dereferencing the NULL pointer.

Add dns_view_getresolver() method which attaches to view->resolver object under the lock (if it still exists) ensuring the dns_resolver will be kept referenced until not needed.

Closes #3560 (closed)