[flang] Fix crash from bug#63099 (gfortran/regression/oldstyle_3.f90)

[klausler]

Summary

In InitElement() in flang/lib/Semantics/data-to-inits.cpp, don't
assume that values_.Location() can be called until after it has been
verified that values_.IsAtEnd() is not true.

Fixes https://github.com/llvm/llvm-project/issues/63099.

flang/lib/Semantics/data-to-inits.cpp

   const Symbol &symbol{offsetSymbol.symbol()};
   const Symbol *lastSymbol{GetLastSymbol(designator)};
   bool isPointer{lastSymbol && IsPointer(*lastSymbol)};
   bool isProcPointer{lastSymbol && IsProcedurePointer(*lastSymbol)};
   evaluate::FoldingContext &context{exprAnalyzer_.GetFoldingContext()};
-  auto &messages{context.messages()};
-  auto restorer{
-      messages.SetLocation(values_.LocateSource().value_or(messages.at()))};
 
   const auto DescribeElement{[&]() {
     if (auto badDesignator{
             evaluate::OffsetToDesignator(context, offsetSymbol)}) {
       return badDesignator->AsFortran();

                  offsetSymbol.offset() + offsetSymbol.size()) > symbol.size()) {
     OutOfRangeError();
     return false;
   }
 
+  auto &messages{context.messages()};
+  auto restorer{
+      messages.SetLocation(values_.LocateSource().value_or(messages.at()))};
   const SomeExpr *expr{*values_};
   if (!expr) {
     CHECK(exprAnalyzer_.context().AnyFatalError());
   } else if (symbol.size() > maxDataInitBytes) {
     evaluate::AttachDeclaration(