BUG/MEDIUM: stream-int: convert some co_data() checks to channel_is_empty()

BUG/MEDIUM: stream-int: convert some co_data() checks to channel_is_empty()

Splicing was in great part broken over the last few development version
due to the use of co_data() to detect if data are available in the channel.
But co_data() only looks at buffered data, not spliced data.

Channel_is_empty() takes care of both and should be used. With this,
splicing restarts to work but there are still a few cases where transfers
may stall.

No backport is needed.