Fix Coverity dereference before null check issue (intel#13456)

Fixes an issue found by Coverity in `addEventForNode` about
dereferencing the `EventImpl` before a null check.

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Co-authored-by: Ben Tracy <ben.tracy@codeplay.com>

sycl/source/detail/graph_impl.hpp

@@ -899,7 +899,7 @@ class graph_impl {
   void addEventForNode(std::shared_ptr<graph_impl> GraphImpl,
                        std::shared_ptr<sycl::detail::event_impl> EventImpl,
                        std::shared_ptr<node_impl> NodeImpl) {
-    if (!(EventImpl->getCommandGraph()))
+    if (EventImpl && !(EventImpl->getCommandGraph()))
       EventImpl->setCommandGraph(GraphImpl);
     MEventsMap[EventImpl] = NodeImpl;
   }