BUG/MINOR: lua: Properly check negative offset in Channel/HttpMessage functions

BUG/MINOR: lua: Properly check negative offset in Channel/HttpMessage functions

In Channel and HTTPMessage classes, several functions uses an offset that
may be negative to start from the end of incoming data. But, after
calculation, the offset must never be negative. However, there is a bug
because of a bad cast to unsigned when "input + offset" is performed. The
result must be a signed integer.

This patch should fix most of defects reported in the issue #1347. It only
affects 2.5-dev. No backport needed.