drm: check for minor master before allowing drop master.

When fast user switching a lot eventually we get to the point,
where we were checking for the wrong thing in this function.
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_stub.c

@@ -185,6 +185,9 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
 	if (!file_priv->is_master)
 		return -EINVAL;
 
+	if (!file_priv->minor->master)
+		return -EINVAL;
+
 	mutex_lock(&dev->struct_mutex);
 	drm_master_put(&file_priv->minor->master);
 	file_priv->is_master = 0;