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>