MINOR: gcc: Fix a silly gcc warning in connect_server()

Don't know why it happens now, but gcc seems to think srv_conn may be NULL when
a reused connection is removed from the orphan list. It happens when HAProxy is
compiled with -O2 with my gcc (8.3.1) on fedora 29... Changing a little how
reuse parameter is tested removes the warnings. So...

This patch may be backported to 1.9.
1 file changed