MINOR: mux-h1: Subscribe for sends if output buffer is not empty in h1_snd_pipe

MINOR: mux-h1: Subscribe for sends if output buffer is not empty in h1_snd_pipe

In h1_snd_pipe(), before sending spliced data, we take care to flush the
output buffer by subscribing for sends. However, the condition to do so is
not accurate. We test data remaining in the pipe. It works but it also
unnecessarily subscribes H1C for sends when the output buffer is empty if we
are unable to send all spliced data in one time. Instead, H1C is now
subscribed for sends if output buffer is not empty.