MINOR: h3: fix possible invalid dereference on htx parsing

The htx variable is only initialized if we have received a HTTP/3
HEADERS frame. Else it must not be dereferenced.

This should fix the compilation on CI with gcc.
  src/h3.c: In function ‘h3_decode_qcs’:
  src/h3.c:224:14: error: ‘htx’ may be used uninitialized in this function
  [-Werror=maybe-uninitialized]
    224 |   htx->flags |= HTX_FL_EOM
1 file changed