[flang][runtime] Detect overflow of fixed-sized exponent output field

[klausler]

Summary

When Ew.dEe or Gw.dEe output has an exponent requiring more than 'e'
digits, the whole output field must overflow to asterisks. The runtime
was detecting short fields and padding them with zeroes, but not
overflow.

flang/runtime/edit-output.h

if (*p == '-' || *p == '+') {
       if (res.length == 1) {
         return false;
       }
       ++p;
     }
     return *p < '0' || *p > '9';
   }
 
+  // Returns null when the exponent overflows a fixed-size output field.
   const char *FormatExponent(int, const DataEdit &edit, int &length);
   bool EmitPrefix(const DataEdit &, std::size_t length, std::size_t width);
   bool EmitSuffix(const DataEdit &);
 
   IoStatementState &io_;
   int trailingBlanks_{0}; // created when Gw editing maps to Fw
   char exponent_[16];
 };

flang/runtime/edit-output.cpp

const char *RealOutputEditingBase::FormatExponent(
     int expo, const DataEdit &edit, int &length) {
   char *eEnd{&exponent_[sizeof exponent_]};
   char *exponent{eEnd};
   for (unsigned e{static_cast<unsigned>(std::abs(expo))}; e > 0;) {
     unsigned quotient{e / 10u};
     *--exponent = '0' + e - 10 * quotient;
     e = quotient;
   }
+  bool overflow{false};
   if (edit.expoDigits) {
     if (int ed{*edit.expoDigits}) { // Ew.dEe with e > 0
+      overflow = exponent + ed < eEnd;
       while (exponent > exponent_ + 2 /*E+*/ && exponent + ed > eEnd) {
         *--exponent = '0';
       }
     } else if (exponent == eEnd) {
       *--exponent = '0'; // Ew.dE0 with zero-valued exponent
     }
   } else { // ensure at least two exponent digits
     while (exponent + 2 > eEnd) {
       *--exponent = '0';
     }
   }
   *--exponent = expo < 0 ? '-' : '+';
   if (edit.expoDigits || edit.IsListDirected() || exponent + 3 == eEnd) {
     *--exponent = edit.descriptor == 'D' ? 'D' : 'E'; // not 'G'
   }
   length = eEnd - exponent;
-  return exponent;
+  return overflow ? nullptr : exponent;
 }
 
 bool RealOutputEditingBase::EmitPrefix(
     const DataEdit &edit, std::size_t length, std::size_t width) {
   if (edit.IsListDirected()) {
     int prefixLength{edit.descriptor == DataEdit::ListDirectedRealPart ? 2
             : edit.descriptor == DataEdit::ListDirectedImaginaryPart   ? 0
                                                                        : 1};
while (true) {
     int trailingZeroes{flags & decimal::Minimize
             ? 0
             : std::max(0,
                   significantDigits - (convertedDigits + zeroesBeforePoint))};
     int totalLength{signLength + digitsBeforePoint + zeroesBeforePoint +
         1 /*'.'*/ + zeroesAfterPoint + digitsAfterPoint + trailingZeroes +
         expoLength};
     int width{editWidth > 0 ? editWidth : totalLength};
-    if (totalLength > width) {
+    if (totalLength > width || !exponent) {
       return io_.EmitRepeated('*', width);
     }
     if (totalLength < width && digitsBeforePoint == 0 &&
         zeroesBeforePoint == 0) {
       zeroesBeforePoint = 1;
       ++totalLength;
     }
     if (totalLength < width && noLeadingSpaces) {