MINOR: mux-h2: stop on non-DATA and non-EOM HTX blocks

We don't want to send such blocks as DATA frames if they were ever to
appear, let's quit when meeting them.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 3cc3984..5585ac2 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -4283,6 +4283,9 @@
 		goto end;
 	}
 
+	if (type != HTX_BLK_DATA && type != HTX_BLK_EOM)
+		goto end;
+
 	/* for DATA and EOM we'll have to emit a frame, even if empty */
 
 	while (1) {