Reset last_writelen to 0 when stream is removed from tree

When stream is removed from tree, stream is either closed, or its
remote flow control window is depleted.  In the latter case, we
schedule this stream as fast as possible if its remote window gets
positive, since it did not sent anything in its turn.  To achieve
this, reset last_writelen to 0 when stream is removed from tree.

lib/nghttp2_stream.c

@@ -166,6 +166,7 @@ static void stream_obq_remove(nghttp2_stream *stream) {
     stream->queued = 0;
     stream->cycle = 0;
     stream->descendant_last_cycle = 0;
+    stream->last_writelen = 0;
 
     if (stream_subtree_active(dep_stream)) {
       return;