BUG/MINOR: channel/htx: Don't alter channel during forward for empty HTX message

BUG/MINOR: channel/htx: Don't alter channel during forward for empty HTX message

In channel_htx_forward() and channel_htx_forward_forever(), if the HTX message
is empty, the underlying buffer may be really empty too. And we have no warranty
the caller will call htx_to_buf() later. And in practice, it is almost never
done. So the channel's buffer must not be altered. Otherwise, the buffer may be
considered as full (data == size) for an empty HTX message and no outgoing data.

This patch must be backported to 1.9.