Potential `nulltpr` dereference in `graph_impl`

[AlexeySachkov]

llvm/sycl/source/detail/graph_impl.cpp

Lines 1014 to 1023 in 83fe1c1

	if (NewEvent != nullptr) {
	NewEvent->setSubmissionTime();
	NewEvent->setHostEnqueueTime();
	}
	ur_result_t Res =
	Queue->getAdapter()
	->call_nocheck<
	sycl::detail::UrApiKind::urCommandBufferEnqueueExp>(
	CommandBuffer, Queue->getHandleRef(), 0, nullptr, &UREvent);
	NewEvent->setHandle(UREvent);

if on 1014 implies that NewEvent could be nullptr. However, we directly dereference it on 1023.

Either 1023 dereference should be made conditional, or if on 1014 should be turned into an assertion.

[EwanC]

tagging @intel/sycl-graphs-reviewers