BUG/MEDIUM: mux-h2: fix the body_len to check when parsing request trailers

BUG/MEDIUM: mux-h2: fix the body_len to check when parsing request trailers

The h2 content-length validation in commit d12edebe4a ("BUG/MAJOR:
mux-h2: detect incomplete transfers on HEADERS frames as well") was
insufficient. The content-length check is still ineffective on request
trailers and it could not work by default due to the fact that the
default body_len is used in h2c_frt_handle_headers() when processing
trailers, instead of passing h2s->body_len, which was necessarily parsed
before reaching trailers. Let's fix this point first, otherwise fixing
the second issue would break trailers.

Many thanks to Pratham Gupta / alchemy1729 for spotting and analyzing
this problem, and for providing a lightweight reproducer to illustrate
the problem!

This fix must be backported to all versions where the fix above was
backported (i.e. all).