message: Fix undefined behavior in FunctionLibraryDefinition::Find().
  With some combinations of compiler options (though surprisingly not just enabling ASAN), taking the address of the first member through a null `std::shared_ptr<FunctionDefAndOpRegistration>` fails.
diff: @@ -1082,7 +1082,12 @@ bool FunctionLibraryDefinition::Contains(const string& func) const {
  const FunctionDef* FunctionLibraryDefinition::Find(const string& func) const {
  tf_shared_lock l(mu_);
  - return &(FindHelper(func)->fdef);
  + auto result = FindHelper(func);
  + if (result) {
  + return &result->fdef;
  + } else {
  + return nullptr;
  + }
  }
  std::shared_ptr<FunctionLibraryDefinition::FunctionDefAndOpRegistration>
  