[flang] Prevent crash in error case: elemental with alternate return

[klausler]

Summary

When an ELEMENTAL subroutine is erroneously declared with alternate return
arguments, don't crash when checking the ranks of the actual arguments.

flang/lib/Semantics/check-call.cpp

 static bool CheckElementalConformance(parser::ContextualMessages &messages,
     const characteristics::Procedure &proc, evaluate::ActualArguments &actuals,
     evaluate::FoldingContext &context) {
   std::optional<evaluate::Shape> shape;
   std::string shapeName;
   int index{0};
   bool hasArrayArg{false};
   for (const auto &arg : actuals) {
-    if (arg && arg.value().Rank() > 0) {
+    if (arg && !arg->isAlternateReturn() && arg->Rank() > 0) {
       hasArrayArg = true;
       break;
     }
   }
   for (const auto &arg : actuals) {
     const auto &dummy{proc.dummyArguments.at(index++)};
     if (arg) {
       if (const auto *expr{arg->UnwrapExpr()}) {