drm/nouveau/uvmm: fix NULL deref unwinding an OP_MAP_SPARSE op

In the Linux kernel, the following vulnerability has been resolved: drm/nouveau/uvmm: fix NULL deref unwinding an OP_MAP_SPARSE op Each bind_job_op is zeroed by kzalloc_obj() in bind_job_op_from_uop(), and the OP_MAP_SPARSE case in nouveau_uvmm_bind_job_submit() only creates a region, so op->ops stays NULL for a successfully processed sparse map. If a later op in the same job fails, the reverse unwind loop revisits that op and calls drm_gpuva_ops_free(&uvmm->base, op->ops) unconditionally. drm_gpuva_ops_free() dereferences its argument right away (list_for_each_entry_safe on &ops->list), so a NULL op->ops oopses. The path is reachable by any render-node fd holder, since NOUVEAU_VM_BIND is DRM_RENDER_ALLOW. Guard the free with IS_ERR_OR_NULL(), as nouveau_uvmm_bind_job_cleanup() already does for the identical free.