BUG/MINOR: mux-fcgi: Be sure to have a connection as session's origin to use it

BUG/MINOR: mux-fcgi: Be sure to have a connection as session's origin to use it

When default parameters are set in a request message, we get the client
connection using the session's origin. But it is not necessarily a
conncection. For instance, for health checks, thanks to recent changes, it may
be a check object. At this step, the client connection may be NULL. Thus, we
must be sure to have a client connection before using it.

This patch must be backported to 2.1.

(cherry picked from commit bb86a0f7be6ba8a248321a59c4b865e01e33f106)
Signed-off-by: Willy Tarreau <w@1wt.eu>