MINOR: mux-h2: make sure any h2s 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 h2s->endp when
trying to access the flags or the CS.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 1ff0a97..624a1bd 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -523,9 +523,16 @@
 static unsigned int h2_settings_max_concurrent_streams = 100;
 static int h2_settings_max_frame_size         = 0;     /* unset */
 
+/* a dummy closed endpoint */
+static const struct cs_endpoint closed_ep = {
+	. cs       = NULL,
+	.flags     = CS_EP_DETACHED,
+};
+
 /* a dmumy closed stream */
 static const struct h2s *h2_closed_stream = &(const struct h2s){
 	.cs        = NULL,
+	.endp      = (struct cs_endpoint *)&closed_ep,
 	.h2c       = NULL,
 	.st        = H2_SS_CLOSED,
 	.errcode   = H2_ERR_STREAM_CLOSED,
@@ -536,6 +543,7 @@
 /* a dmumy closed stream returning a PROTOCOL_ERROR error */
 static const struct h2s *h2_error_stream = &(const struct h2s){
 	.cs        = NULL,
+	.endp      = (struct cs_endpoint *)&closed_ep,
 	.h2c       = NULL,
 	.st        = H2_SS_CLOSED,
 	.errcode   = H2_ERR_PROTOCOL_ERROR,
@@ -546,6 +554,7 @@
 /* a dmumy closed stream returning a REFUSED_STREAM error */
 static const struct h2s *h2_refused_stream = &(const struct h2s){
 	.cs        = NULL,
+	.endp      = (struct cs_endpoint *)&closed_ep,
 	.h2c       = NULL,
 	.st        = H2_SS_CLOSED,
 	.errcode   = H2_ERR_REFUSED_STREAM,
@@ -556,6 +565,7 @@
 /* and a dummy idle stream for use with any unannounced stream */
 static const struct h2s *h2_idle_stream = &(const struct h2s){
 	.cs        = NULL,
+	.endp      = (struct cs_endpoint *)&closed_ep,
 	.h2c       = NULL,
 	.st        = H2_SS_IDLE,
 	.errcode   = H2_ERR_STREAM_CLOSED,