phoc tries to send wl_touch.down for an already destroyed subsurface

It appears that gtk#20 and gnome-control-center#110 (closed) are caused by wl_touch.down message with empty surface:

[698420.747] wl_touch@23.down(356, 849452, nil, 0, 12.000000, 23.000000)

It looks like it's caused by trying to send it to a subsurface that gtk already called destroy on. It can be reproduced by repeatedly tapping at the beginning of an empty gtk entry.