BUG/MINOR: mux-h2: only update rxbuf's length for H1 headers

BUG/MINOR: mux-h2: only update rxbuf's length for H1 headers

In h2c_decode_headers() we update the buffer's length according to the
amount of data produced (outlen). But in case of HTX this outlen value
is not a quantity, just an indicator of success, resulting in the buffer
being added one extra byte and temporarily showing .data > .size, which
is wrong. Fortunately this is overridden when leaving the function by
htx_to_buf() so the impact only exists in step-by-step debugging, but
it definitely needs to be fixed.

This must be backported to 1.9.