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

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.

Differential Revision: https://reviews.llvm.org/D145745

flang/runtime/format-implementation.h

@@ -212,7 +212,7 @@ static void HandleControl(CONTEXT &context, char ch, char next, int n) {
 // 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;
@@ -267,7 +267,6 @@ int FormatControl<CONTEXT>::CueUpNextDataEdit(Context &context, bool stop) {
       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
@@ -305,12 +304,13 @@ int FormatControl<CONTEXT>::CueUpNextDataEdit(Context &context, bool stop) {
               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;