libcheese: Fix critical on failing to load cropping cursor

Loading a cursor can fail (`gdk_cursor_new_for_display()` can return
`NULL`); avoid a critical warning in that case.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
1 job for master in 5 minutes and 35 seconds
Status Job ID Name Coverage
  Test
passed #917659
build-fedora

00:05:35