bounds-check dns response parsing against packet end

[rootvector2]

parseQname follows a compression pointer and copies label bytes with no packet-end check, and the TXT loop in parseQuestionOrAnswer underflows rdlength when a sub-string length exceeds it, so a malformed DNS response read from the resolver's UDP socket reads past the packet buffer.

[rootvector2]

@phoddie

[phoddie]

Thank you for the PR.

• It identifies some memory safety issues. That's great.
• It missed some others - most notably the xs_dnspacket_get_* functions don't validate that the header is fully present.
• Some of the changes to optimize refresh of the packet pointer will break, because packet can move during certain operations
• Some of fixes are more invasive / complicate than strictly necessary.

I drafted an alternative solution. You can see that as a comparison with the current Moddable SDK version in this gist.