xwayland: fix segfault when running x11perf

Running "x11perf -reps 1 -repeat 1 -all" will segfault, due to
windows being left on lists after being destroyed.

The fix was devised after running Weston under valgrind whilst
x11perf was running.

Fixes: wayland/weston#797



Signed-off-by: Brendan King <Brendan.King@imgtec.com>