drm/xe/selftests: Fix an error pointer dereference bug

Check if "bo" is an error pointer before calling xe_bo_lock() on it.

Fixes: d6abc18d ("drm/xe/xe2: Modify xe_bo_test for system memory")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
(cherry picked from commit 88ec2352)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

drivers/gpu/drm/xe/tests/xe_bo.c

@@ -125,14 +125,13 @@ static void ccs_test_run_tile(struct xe_device *xe, struct xe_tile *tile,
 
 	bo = xe_bo_create_user(xe, NULL, NULL, SZ_1M, DRM_XE_GEM_CPU_CACHING_WC,
 			       ttm_bo_type_device, bo_flags);
-
-	xe_bo_lock(bo, false);
-
 	if (IS_ERR(bo)) {
 		KUNIT_FAIL(test, "Failed to create bo.\n");
 		return;
 	}
 
+	xe_bo_lock(bo, false);
+
 	kunit_info(test, "Verifying that CCS data is cleared on creation.\n");
 	ret = ccs_test_migrate(tile, bo, false, 0ULL, 0xdeadbeefdeadbeefULL,
 			       test);