When resquery_response() was called with ISC_R_SHUTTINDOWN, the region
argument would be NULL, but rctx_respinit() would try to pass
region->base and region->len to the isc_buffer_init() leading to
a NULL pointer dereference. Properly handle non-ISC_R_SUCCESS by
ignoring the provided region.
(cherry picked from commit 93259812dd)
4bf253ffe1