[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

   IostatUTF8Decoding,
   IostatUnitOverflow,
   IostatBadRealInput,
+  IostatBadScaleFactor,
 };
 
 const char *IostatErrorString(int);

flang/runtime/edit-output.cpp

   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);
   }

flang/runtime/iostat.cpp

     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;
   }