BUG/MEDIUM: mux-fcgi: Fix locking of idle_conns lock in the FCGI I/O callback

BUG/MEDIUM: mux-fcgi: Fix locking of idle_conns lock in the FCGI I/O callback

When the commit e388f2fbc ("MEDIUM: muxes: mark idle conns tasklets with
TASK_F_USR1") was backported (commit id b360bb88 on the 2.3), a call to
HA_SPIN_UNLOCK() was missed and not moved in the right code block. Thus it
is possible to unlock the idle_conns lock while it was never acquired.

This patch should fix the issue #1191. It is 2.3 specific, thus there is no
upstream commid id. No backport is needed except if commit b360bb88 is
backported.