[flang][runtime] Fix return value for MINVAL/MAXVAL for CHARACTER(kind > 1)

[klausler]

Summary

CharacterExtremumAccumulator::GetResult() needs to use byte counts, not wide
character counts, when calling memcpy() & memset().

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/runtime/extrema.cpp

 template <int KIND, bool IS_MAXVAL> class CharacterExtremumAccumulator {
 public:
   using Type = CppTypeFor<TypeCategory::Character, KIND>;
   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_, charLen_);
+      std::memcpy(p, extremum_, byteSize);
     } else {
-      // empty array: result is all zero-valued characters
-      std::memset(p, 0, charLen_);
+      // 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);
+      }
     }
   }
   bool Accumulate(const Type *x) {
     if (!extremum_) {
       extremum_ = x;
     } else {
       int cmp{CharacterScalarCompare(x, extremum_, charLen_, charLen_)};
       if (IS_MAXVAL == (cmp > 0)) {