Ignore upcall_flush for channels that are disassociated from ports. This makes task-comm-10 break a little less hard, but it still leaks because messages pending in the channel are never freed.

src/rt/circular_buffer.cpp

@@ -142,3 +142,8 @@ bool
 circular_buffer::is_empty() {
     return _unread == 0;
 }
+
+size_t
+circular_buffer::size() {
+    return _unread;
+}