rlm_kafka: unbox async opaques with talloc_get_type_abort

rlm_kafka: unbox async opaques with talloc_get_type_abort

xctx->inst / xctx->thread / msg->_private are all void pointers that
we know at the call-site should point back at our typed talloc chunks.
Using talloc_get_type_abort turns a subtle type-system hole into a
loud abort at the exact callsite if the opaque ever gets crossed over,
instead of a mystery crash deep in the function body.

The DR callback keeps its NULL-guard first - a NULL opaque is the
documented signal for fire-and-forget produces, so it's part of the
contract, not an error worth asserting on.