[flang][runtime] Catch infinite unlimited format repetition better

[klausler]

Summary

The runtime check for infinite unlimited format repetition is missing
the case of implicit unlimited format repetition at the top level
without finding the next data edit descriptor that corresponds with
a data list item.

Replace the check with a Boolean flag that detects unlimited
repetition hitting a right parenthesis and restarting, and fail
when it happens the second time.

flang/runtime/format-implementation.h

 }
 
 // Locates the next data edit descriptor in the format.
 // Handles all repetition counts and control edit descriptors.
 // Generally assumes that the format string has survived the common
 // format validator gauntlet.
 template <typename CONTEXT>
 int FormatControl<CONTEXT>::CueUpNextDataEdit(Context &context, bool stop) {
-  int unlimitedLoopCheck{-1};
+  bool hitUnlimitedLoopEnd{false};
   // Do repetitions remain on an unparenthesized data edit?
   while (height_ > 1 && format_[stack_[height_ - 1].start] != '(') {
     offset_ = stack_[height_ - 1].start;
     int repeat{stack_[height_ - 1].remaining};
     --height_;
     if (repeat > 0) {
       return repeat;
     }
if (ch == '(') {
             "FORMAT stack overflow: too many nested parentheses",
             maybeReversionPoint);
         return 0;
       }
       stack_[height_].start = offset_ - 1; // the '('
       RUNTIME_CHECK(context, format_[stack_[height_].start] == '(');
       if (unlimited || height_ == 0) {
         stack_[height_].remaining = Iteration::unlimited;
-        unlimitedLoopCheck = offset_ - 1;
       } else if (repeat) {
         if (*repeat <= 0) {
           *repeat = 1; // error recovery
         }
         stack_[height_].remaining = *repeat - 1;
       } else {
         stack_[height_].remaining = 0;
       }
if (ch == '(') {
       if (stack_[height_ - 1].remaining == Iteration::unlimited) {
         if (height_ > 1 && GetNextChar(context) != ')') {
           ReportBadFormat(context,
               "Unlimited repetition in FORMAT may not be followed by more "
               "items",
               restart);
           return 0;
         }
-        if (offset_ == unlimitedLoopCheck) {
+        if (hitUnlimitedLoopEnd) {
           ReportBadFormat(context,
               "Unlimited repetition in FORMAT lacks data edit descriptors",
               restart);
           return 0;
         }
+        hitUnlimitedLoopEnd = true;
         offset_ = restart;
       } else if (stack_[height_ - 1].remaining-- > 0) {
         offset_ = restart;
       } else {
         --height_;
       }
     } else if (ch == '\'' || ch == '"') {
       // Quoted 'character literal'