BUG/MINOR: mux-fcgi: Be sure to have a connection to unsubcribe

When the mux is released, It must own the connection to unsubcribe.
This patch fixes the issue #283. No backport needed.
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index d2115ac..ea8e48b 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -576,7 +576,7 @@
 		}
 		if (fconn->wait_event.tasklet)
 			tasklet_free(fconn->wait_event.tasklet);
-		if (fconn->wait_event.events != 0)
+		if (conn && fconn->wait_event.events != 0)
 			conn->xprt->unsubscribe(conn, conn->xprt_ctx, fconn->wait_event.events,
 						&fconn->wait_event);
 	}