BUG/MINOR: h3: fix TRAILERS encoding

BUG/MINOR: h3: fix TRAILERS encoding

HTTP/3 trailers encoding was never working as intended. It's because
h3_trailers_to_htx() manipulate a newly allocated buffer instead of the
already existing channel one. Thus, HTX message handled by the stream
was incomplete as it lacked trailers and EOM.

Fix this by reusing the already allocated channel buffer in
h3_trailers_to_htx().

This bug was detected by simulating TRAILERS emission which generate
CL--- state due to missing request side termination signal. Its impact
is deemed as minimal as trailers are pretty infrequent for now in
HTTP/3.

This must be backported up to 2.7.