MEDIUM: frontend: don't restrict frontend_accept() to connections anymore

Now it can also initialize streams initiated by applets. This will be
needed for HTTP/2.
diff --git a/src/stream.c b/src/stream.c
index 5a021aa..59d1d5d 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -222,7 +222,7 @@
 	else if (appctx)
 		s->si[0].flags |= SI_FL_WAIT_DATA;
 
-	if (conn && p->accept && p->accept(s) < 0)
+	if (p->accept && p->accept(s) < 0)
 		goto out_fail_accept;
 
 	if (conn) {