present: fix freed pointer access

When a vblank has been marked as aborted, it's going to be free in the
flip_notify function when stopped. We can't notify it after it's
stopped because the pointer is invalid.

Valgrind backtrace:

==5331== Invalid read of size 8
==5331==    at 0x212B4D: present_vblank_notify (present_vblank.c:34)
==5331==    by 0x21439B: present_wnmd_flip_notify (present_wnmd.c:194)
==5331==    by 0x21439B: present_wnmd_event_notify (present_wnmd.c:228)
==5331==    by 0x156216: xwl_present_sync_callback (xwayland-present.c:282)
==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==  Address 0x1b44dc98 is 40 bytes inside a block of size 184 free'd
==5331==    at 0x48369EB: free (vg_replace_malloc.c:530)
==5331==    by 0x213B0A: present_wnmd_free_idle_vblanks (present_wnmd.c:118)
==5331==    by 0x213B0A: present_wnmd_flips_stop (present_wnmd.c:161)
==5331==    by 0x2143EF: present_wnmd_flip_notify (present_wnmd.c:192)
==5331==    by 0x2143EF: present_wnmd_event_notify (present_wnmd.c:228)
==5331==    by 0x156216: xwl_present_sync_callback (xwayland-present.c:282)
==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==  Block was alloc'd at
==5331==    at 0x48377D5: calloc (vg_replace_malloc.c:711)
==5331==    by 0x212D9F: present_vblank_create (present_vblank.c:69)
==5331==    by 0x214014: present_wnmd_pixmap (present_wnmd.c:610)
==5331==    by 0x21576C: proc_present_pixmap (present_request.c:150)
==5331==    by 0x27599D: Dispatch (dispatch.c:479)
==5331==    by 0x279945: dix_main (main.c:276)
==5331==    by 0x633AB16: (below main) (libc-start.c:310)

v2: Still notify aborted flips (Roman)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107314


Reviewed-by: Roman Gilg <subdiff@gmail.com>
Tested-by: Roman Gilg <subdiff@gmail.com>