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.