wayland: check parent surface for subsurface

wayland: check parent surface for subsurface

When an event is received while a tooltip is showing, the GtkTooltip's
event handling code can end up calling gdk_window_set_transient_for()
from gtk_tooltip_set_last_window().

The Wayland GDK backend will try to automatically create a subsurface
in gdk_wayland_window_set_transient_for() but if the parent surface is
gone meanwhile, this will will cause a crash when trying to create a
subsurface from a parent with a null surface.

Checking for the parent is not sufficient, we ought to check for the
parent surface as well to avoid the crash.

https://bugzilla.gnome.org/show_bug.cgi?id=782283