Properly handle ISC_R_SHUTTINGDOWN in resquery_response()

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.

Closes #3960 (closed)

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