drm/r600: fix possible NULL pointer derefernce

Reported-by: Alexander Y. Fomichev <git.user@gmail.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/r600_blit.c

@@ -538,9 +538,12 @@ int
 r600_prepare_blit_copy(struct drm_device *dev, struct drm_file *file_priv)
 {
 	drm_radeon_private_t *dev_priv = dev->dev_private;
+	int ret;
 	DRM_DEBUG("\n");
 
-	r600_nomm_get_vb(dev);
+	ret = r600_nomm_get_vb(dev);
+	if (ret)
+		return ret;
 
 	dev_priv->blit_vb->file_priv = file_priv;