MINOR: mux-h2: consistently rely on the htx variable to detect the mode

MINOR: mux-h2: consistently rely on the htx variable to detect the mode

In h2_frt_transfer_data(), we support both HTX and legacy modes. The
HTX mode is detected from the proxy option and sets a valid pointer
into the htx variable. Better rely on this variable in all the function
rather than testing the option again. This way the code is clearer and
even the compiler knows this pointer is valid when it's dereferenced.

This should be backported to 1.9 if the b_is_null() patch is backported.