BUG/MEDIUM: mux-spop: Properly detect truncated frames on demux to report error

BUG/MEDIUM: mux-spop: Properly detect truncated frames on demux to report error

There was no test in the demux part to detect truncated frames and to report
an error at the connection level. The SPOP streams were properly switch to
half-closed state. But waiting the associated SPOE applets were woken up and
released, the SPOP connection could be woken up several times for nothing. I
never triggered the watchdog in that case, but it is not excluded.

Now, at the end of the demux function, if a specific test was added to
detect truncated frames to report an error and close the connection.

This patch must be backported to 3.1.

(cherry picked from commit 16314bb93c2c2c47c648da880ab55bf2ac01d9d4)
[wt: update ctx]
Signed-off-by: Willy Tarreau <w@1wt.eu>