present/wnmd: Relax assertion on CRTC on abort_vblank()

Currently, the function `present_wnmd_abort_vblank()` would fail if the
given `crtc` is NULL.

However, `xwl_present_get_crtc()` can return `NULL` under some
circumstances, which would cause an unexpected termination of Xwayland
in such a case, caused by the assertion failure being triggered.

Remove the assertion, considering that the `crtc` isn't actually used in
neither `present_wnmd_abort_vblank()` nor `xwl_present_abort_vblank()`.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Closes: https://gitlab.freedesktop.org/xorg/xserver/issues/937
(cherry picked from commit 4f984fc0)