BUG/MINOR: h3: reset HTX message after trailers parsing error

BUG/MINOR: h3: reset HTX message after trailers parsing error

HTTP/3 trailers parsing was adapted by the following patch to rely on
the common function http_trailers_to_htx().

  25a11d345425be08cb01fec0eae5dfe8b423ee57
  MEDIUM: h3: rely on the generic trailers parser

Along this change, h3_trailers_to_htx() error path has been updated to
regroup failure with or without glitch via two labels. However, it
bypasses HTX message reset, leaving the stream buffer appearing as full
and possibly containing a partial trailers conversion.

Fix this by restoring the single unified function end. Return value and
glitch are directly adjusted in the switch statement from
http_trailers_to_htx() result.

No need to backport.