drm/radeon: fix a possible null pointer dereference

In radeon_tv_get_modes(), the return value of drm_cvt_mode()
is assigned to mode, which will lead to a NULL pointer
dereference on failure of drm_cvt_mode(). Add a check to
avoid null point dereference.
Signed-off-by: Ma Ke <make_ruc2021@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_connectors.c

@@ -1119,6 +1119,8 @@ static int radeon_tv_get_modes(struct drm_connector *connector)
 	else {
 		/* only 800x600 is supported right now on pre-avivo chips */
 		tv_mode = drm_cvt_mode(dev, 800, 600, 60, false, false, false);
+		if (!tv_mode)
+			return 0;
 		tv_mode->type = DRM_MODE_TYPE_DRIVER | DRM_MODE_TYPE_PREFERRED;
 		drm_mode_probed_add(connector, tv_mode);
 	}