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 cursor-critical in 6 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #917595
build-fedora

00:06:57