Don't reprioritize stream if it is not in dependency tree

Normally stream is in dependency tree, but sometimes it isn't.  For
example, client does not put pushed reserved stream in dependency
tree.

lib/nghttp2_session.c

@@ -573,6 +573,10 @@ int nghttp2_session_reprioritize_stream
   nghttp2_stream *dep_stream;
   nghttp2_stream *root_stream;
 
+  if(!nghttp2_stream_in_dep_tree(stream)) {
+    return 0;
+  }
+
   if(pri_spec->stream_id == stream->stream_id) {
     return nghttp2_session_terminate_session_with_reason
       (session, NGHTTP2_PROTOCOL_ERROR, "depend on itself");