MINOR: mux-fcgi: make sure any stream always has an endpoint

The principle that each mux stream should have an endpoint is not
guaranteed for closed streams that map to the dummy static streams.
Let's have a dummy endpoint for use with such streams. It only has
the DETACHED flag and a NULL conn_stream, and is referenced by all
the closed streams so that we can afford not to test strm->endp when
trying to access the flags or the CS.
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index 2afa2ff..ab3d013 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -371,9 +371,16 @@
 static void fcgi_strm_alert(struct fcgi_strm *fstrm);
 static int fcgi_strm_send_abort(struct fcgi_conn *fconn, struct fcgi_strm *fstrm);
 
+/* a dummy closed endpoint */
+static const struct cs_endpoint closed_ep = {
+	. cs       = NULL,
+	.flags     = CS_EP_DETACHED,
+};
+
 /* a dmumy management stream */
 static const struct fcgi_strm *fcgi_mgmt_stream = &(const struct fcgi_strm){
 	.cs        = NULL,
+	.endp      = (struct cs_endpoint*)&closed_ep,
 	.fconn     = NULL,
 	.state     = FCGI_SS_CLOSED,
 	.flags     = FCGI_SF_NONE,
@@ -383,6 +390,7 @@
 /* and a dummy idle stream for use with any unknown stream */
 static const struct fcgi_strm *fcgi_unknown_stream = &(const struct fcgi_strm){
 	.cs        = NULL,
+	.endp      = (struct cs_endpoint*)&closed_ep,
 	.fconn     = NULL,
 	.state     = FCGI_SS_IDLE,
 	.flags     = FCGI_SF_NONE,