BUG/MINOR: h3: fix memleak on HEADERS parsing failure

BUG/MINOR: h3: fix memleak on HEADERS parsing failure

If an error is triggered on H3 HEADERS parsing, the allocated buffer for
HTX data is not freed.

To prevent this memleak, all return path have been centralized using
goto statements.

Also, as a small bonus, offer_buffers() is not called anymore if buffer
is not freed because sedesc has taken it. However this change has
probably no noticeable effect as dynamic buffers management is not
functional currently.

This should be backported up to 2.6.