[linux-port] Varargs in StringCchPrintfA (#1421)

The StringCchPrintfA implementation for Unix systems mistakenly
used the wrong printf variants. Instead of accessing the variadic
arguments, it was just using the va_list variable as a format
parameter. Additionally, reusing va_list has undefined results.
So a copy needs to be made using va_copy.

lib/DxcSupport/WinFunctions.cpp

@@ -22,14 +22,17 @@
 HRESULT StringCchPrintfA(char *dst, size_t dstSize, const char *format, ...) {
   va_list args;
   va_start(args, format);
+  va_list argscopy;
+  va_copy(argscopy, args);
   // C++11 snprintf can return the size of the resulting string if it was to be
   // constructed.
-  size_t size = snprintf(nullptr, 0, format, args) + 1; // Extra space for '\0'
+  size_t size = vsnprintf(nullptr, 0, format, argscopy) + 1; // Extra space for '\0'
   if (size > dstSize) {
     *dst = '\0';
   } else {
-    snprintf(dst, size, format, args);
+    vsnprintf(dst, size, format, args);
   }
+  va_end(argscopy);
   va_end(args);
   return S_OK;
 }