[flang][runtime] Fix MINVAL([CHARACTER(2)::])

[klausler]

Summary

The result of MINVAL over an empty default character array should
(per the standard) have a 127 in every character position, not just
the first.

[vdonaldson]

nit: The comment assumes MINVAL, although the code is also invoked for MAXVAL.

flang/runtime/extrema.cpp

explicit CharacterExtremumAccumulator(const Descriptor &array)
       : array_{array}, charLen_{array_.ElementBytes() / KIND} {}
   void Reinitialize() { extremum_ = nullptr; }
   template <typename A> void GetResult(A *p, int /*zeroBasedDim*/ = -1) const {
     static_assert(std::is_same_v<A, Type>);
     std::size_t byteSize{array_.ElementBytes()};
     if (extremum_) {
       std::memcpy(p, extremum_, byteSize);
     } else {
-      // empty array
-      if constexpr (KIND == 1) { // ASCII
-        *p = IS_MAXVAL ? 0 : 127; // 127 required by standard
-      } else {
-        std::memset(p, IS_MAXVAL ? 0 : 255, byteSize);
-      }
+      // Empty array; fill with character 0 for MAXVAL.
+      // For MINVAL, fill with 127 if ASCII as required
+      // by the standard, otherwise set all of the bits.
+      std::memset(p, IS_MAXVAL ? 0 : KIND == 1 ? 127 : 255, byteSize);
     }
   }
   bool Accumulate(const Type *x) {
     if (!extremum_) {
       extremum_ = x;
     } else {
       int cmp{CharacterScalarCompare(x, extremum_, charLen_, charLen_)};
       if (IS_MAXVAL == (cmp > 0)) {