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: #797 (closed)

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