[flang][runtime] Catch input error on missing NAMELIST scalar

[klausler]

Summary

While a NAMELIST input group item for an array can have fewer
values than expected, or none, an input group item for a scalar
must have a value.

flang/runtime/io-stmt.h

 class ListDirectedStatementState<Direction::Input>
     : public FormattedIoStatementState<Direction::Input> {
 public:
+  bool inNamelistArray() const { return inNamelistArray_; }
+  void set_inNamelistArray(bool yes = true) { inNamelistArray_ = yes; }
+
   // Skips value separators, handles repetition and null values.
   // Vacant when '/' appears; present with descriptor == ListDirectedNullValue
   // when a null value appears.

   // input statement.  This member function resets some state so that
   // repetition and null values work correctly for each successive
   // NAMELIST input item.
-  void ResetForNextNamelistItem() {
+  void ResetForNextNamelistItem(bool inNamelistArray) {
     remaining_ = 0;
     eatComma_ = false;
     realPart_ = imaginaryPart_ = false;
+    inNamelistArray_ = inNamelistArray;
   }
 
 private:

   bool hitSlash_{false}; // once '/' is seen, nullify further items
   bool realPart_{false};
   bool imaginaryPart_{false};
+  bool inNamelistArray_{false};
 };
 
 template <Direction DIR>

flang/runtime/namelist.cpp

     }
     io.HandleRelativePosition(byteCount);
     // Read the values into the descriptor.  An array can be short.
-    listInput->ResetForNextNamelistItem();
+    listInput->ResetForNextNamelistItem(useDescriptor->rank() > 0);
     if (!descr::DescriptorIO<Direction::Input>(io, *useDescriptor)) {
       return false;
     }

 }
 
 bool IsNamelistName(IoStatementState &io) {
-  if (io.get_if<ListDirectedStatementState<Direction::Input>>()) {
-    if (io.mutableModes().inNamelist) {
+  if (auto *listInput{
+          io.get_if<ListDirectedStatementState<Direction::Input>>()}) {
+    if (listInput->inNamelistArray()) {
       SavedPosition savedPosition{io};
       std::size_t byteCount{0};
       if (auto ch{io.GetNextNonBlank(byteCount)}) {