xwayland: fix access to invalid pointer

xwl_output->randr_crtc is used in the update_screen_size() function :

==5331== Invalid read of size 4
==5331==    at 0x15263D: update_screen_size (xwayland-output.c:190)
==5331==    by 0x152C48: xwl_output_remove (xwayland-output.c:413)
==5331==    by 0x6570FCD: ffi_call_unix64 (in /usr/lib/x86_64-linux-gnu/libffi.so.6.0.4)
==5331==    by 0x657093E: ffi_call (in /usr/lib/x86_64-linux-gnu/libffi.so.6.0.4)
==5331==    by 0x4DDB183: ??? (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x4DD79D8: ??? (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x4DD8EA3: wl_display_dispatch_queue_pending (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x14BCCA: xwl_read_events (xwayland.c:814)
==5331==    by 0x2AC0D0: ospoll_wait (ospoll.c:651)
==5331==    by 0x2A5322: WaitForSomething (WaitFor.c:208)
==5331==    by 0x27574B: Dispatch (dispatch.c:421)
==5331==    by 0x279945: dix_main (main.c:276)
==5331==  Address 0x1aacb5f4 is 36 bytes inside a block of size 154 free'd
==5331==    at 0x48369EB: free (vg_replace_malloc.c:530)
==5331==    by 0x1F8AE8: RROutputDestroyResource (rroutput.c:421)
==5331==    by 0x29A2AC: doFreeResource (resource.c:880)
==5331==    by 0x29AE5B: FreeResource (resource.c:910)
==5331==    by 0x152BE0: xwl_output_remove (xwayland-output.c:408)
==5331==    by 0x6570FCD: ffi_call_unix64 (in /usr/lib/x86_64-linux-gnu/libffi.so.6.0.4)
==5331==    by 0x657093E: ffi_call (in /usr/lib/x86_64-linux-gnu/libffi.so.6.0.4)
==5331==    by 0x4DDB183: ??? (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x4DD79D8: ??? (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x4DD8EA3: wl_display_dispatch_queue_pending (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x14BCCA: xwl_read_events (xwayland.c:814)
==5331==    by 0x2AC0D0: ospoll_wait (ospoll.c:651)
==5331==  Block was alloc'd at
==5331==    at 0x48357BF: malloc (vg_replace_malloc.c:299)
==5331==    by 0x1F93E0: RROutputCreate (rroutput.c:83)
==5331==    by 0x152A75: xwl_output_create (xwayland-output.c:361)
==5331==    by 0x14BE59: registry_global (xwayland.c:764)
==5331==    by 0x6570FCD: ffi_call_unix64 (in /usr/lib/x86_64-linux-gnu/libffi.so.6.0.4)
==5331==    by 0x657093E: ffi_call (in /usr/lib/x86_64-linux-gnu/libffi.so.6.0.4)
==5331==    by 0x4DDB183: ??? (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x4DD79D8: ??? (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x4DD8EA3: wl_display_dispatch_queue_pending (in /usr/lib/x86_64-linux-gnu/libwayland-client.so.0.3.0)
==5331==    by 0x14BCCA: xwl_read_events (xwayland.c:814)
==5331==    by 0x2AC0D0: ospoll_wait (ospoll.c:651)
==5331==    by 0x2A5322: WaitForSomething (WaitFor.c:208)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>