qxl_bo_unref() is already performing a NULL check.
Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181120162004.22807-1-cfergeau@redhat.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>