BUG/MEDIUM: mux-fcgi: Truly drain outgoing HTX data when the stream is closed

BUG/MEDIUM: mux-fcgi: Truly drain outgoing HTX data when the stream is closed

When we try to send data to the server and the stream is closed (in error,
in half-closed state or fully closed), remaining data must be drained. This
way the upper stream is able to properly handle the stream close.

However, there was a bug here. The mux claimed to have consumed these data
without draining them from the buffer. So the upper stream will try to send
these data in loop. Because of this bug, it is possible to trigger the
watchdog with a bogus stream.

This patch should fix the issue #3425. It must be backported to all
supported versions.