predict module can get stuck

Refs:

• https://lists.nic.cz/pipermail/knot-dns-users/2017-February/001050.html
• https://gitter.im/CZ-NIC/knot-resolver?at=585e7766c895451b751765fd

I should include the error to make this issue easier to find:

predict.lua:34: 'struct rr_type' has no member named 'nil'

Another report: https://forum.turris.cz/t/kresd-lua-script-mistake/3654

So far it seems this issue doesn't truly break resolving, only the prefetching feature (probably).

mentioned in issue #181 (closed)

@vkriz This is one more candidate, with priority lower than the DNS64 fix.

assigned to @vkriz

changed milestone to %1.3.x

mentioned in commit 97964834

closed via commit 97964834

closed via commit b8399990

closed via merge request !330 (merged)

mentioned in commit b8399990