[flang][runtime] Enforce some limits on kP scale factors

[klausler]

Summary

Ew.d and Dw.d output edit descriptors should respect limitations from
the standard on the value of a kP scale factor with respect to the
digit count (d), at least for values of k other than zero.

flang/include/flang/Runtime/iostat.h

enum Iostat {
   IostatChildInputFromOutputParent,
   IostatChildOutputToInputParent,
   IostatShortRead,
   IostatMissingTerminator,
   IostatBadUnformattedRecord,
   IostatUTF8Decoding,
   IostatUnitOverflow,
   IostatBadRealInput,
+  IostatBadScaleFactor,
 };
 
 const char *IostatErrorString(int);
 
 } // namespace Fortran::runtime::io
 #endif // FORTRAN_RUNTIME_IOSTAT_H_

flang/runtime/edit-output.cpp

if (edit.digits.has_value()) { // E0.d
           sizeof buffer_ - 5; // sign, NUL, + 3 extra for EN scaling
     }
   }
   bool isEN{edit.variation == 'N'};
   bool isES{edit.variation == 'S'};
   int scale{isEN || isES ? 1 : edit.modes.scale}; // 'kP' value
   int zeroesAfterPoint{0};
   if (scale < 0) {
+    if (scale <= -editDigits) {
+      io_.GetIoErrorHandler().SignalError(IostatBadScaleFactor,
+          "Scale factor (kP) %d cannot be less than -d (%d)", scale,
+          -editDigits);
+      return false;
+    }
     zeroesAfterPoint = -scale;
     significantDigits = std::max(0, significantDigits - zeroesAfterPoint);
   } else if (scale > 0) {
+    if (scale >= editDigits + 2) {
+      io_.GetIoErrorHandler().SignalError(IostatBadScaleFactor,
+          "Scale factor (kP) %d cannot be greater than d+2 (%d)", scale,
+          editDigits + 2);
+      return false;
+    }
     ++significantDigits;
     scale = std::min(scale, significantDigits + 1);
   }
   // In EN editing, multiple attempts may be necessary, so it's in a loop.
   while (true) {
     decimal::ConversionToDecimalResult converted{
         Convert(significantDigits, edit.modes.round, flags)};
     if (IsInfOrNaN(converted)) {

flang/runtime/iostat.cpp

const char *IostatErrorString(int iostat) {
   case IostatBadUnformattedRecord:
     return "Erroneous unformatted sequential file record structure";
   case IostatUTF8Decoding:
     return "UTF-8 decoding error";
   case IostatUnitOverflow:
     return "UNIT number is out of range";
   case IostatBadRealInput:
     return "Bad REAL input value";
+  case IostatBadScaleFactor:
+    return "Bad REAL output scale factor (kP)";
   default:
     return nullptr;
   }
 }
 
 } // namespace Fortran::runtime::io