BUG/MAJOR: mux-fcgi: Stop sending loop if FCGI stream is blocked for any reason

BUG/MAJOR: mux-fcgi: Stop sending loop if FCGI stream is blocked for any reason

Because of a typo error in conditions to exit the sending loop, it is possible
to loop infinitely in fcgi_snd_buf() function. Instead of checking the FCGI
stream is not blocked to continue sending data, the FCGI connection is used. So
it is possible to have a stream blocked because there is not enough space in the
mux buffers to copy more data but continue to loop to send more data.

This patch should fix the issue #637. It must be backported to 2.1.

(cherry picked from commit fe410d685ad8bce7399e50695098b69aea07e558)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>