BUG/MINOR: mux-spop: Don't open new streams for SPOP connection on error

BUG/MINOR: mux-spop: Don't open new streams for SPOP connection on error

Till now, only SPOP connections fully closed or those with a TCP connection on
error were concerned. But available streams could be reported for SPOP
connections in error or closing state. But in these states, no NOTIFY frames
will be sent and no ACK frames will be parsed. So, no new SPOP streams should be
opened.

This patch should be backported to 3.1.

(cherry picked from commit 197eaaadfd636441ca6cc5d3e35c52197537bbd6)
Signed-off-by: Willy Tarreau <w@1wt.eu>