BUG/MINOR: h1-htx: properly initialize the err_pos field

BUG/MINOR: h1-htx: properly initialize the err_pos field

Trailers are parsed using a temporary h1m struct, likely due to using
distinct h1 parser states. However, the err_pos field that's used to
decide whether or not to enfore option accept-invalid-http-request (or
response) was not initialized in this struct, resulting in using a
random value that may randomly accept or reject a few bad chars. The
impact is very limited in trailers (e.g. no message size is transmitted
there) but we must make sure that the option is respected, at least for
users facing the need for this option there.

The issue was introduced in 2.0 by commit 2d7c5395ed ("MEDIUM: htx:
Add the parsing of trailers of chunked messages"), and the code moved
from mux_h1.c to h1_htx.c in 2.1 with commit 4f0f88a9d0 ("MEDIUM:
mux-h1/h1-htx: move HTX convertion of H1 messages in dedicated file")
so the patch needs to be backported to all stable versions, and the
file adjusted for 2.0.