Common/StringUtil: Fix more incorrect usage of va_list

src/common/string.cpp

@@ -328,7 +328,10 @@ void String::AppendFormattedStringVA(const char* FormatString, va_list ArgPtr)
 
   for (;;)
   {
-    int ret = std::vsnprintf(pBuffer, currentBufferSize, FormatString, ArgPtr);
+    va_list ArgPtrCopy;
+    va_copy(ArgPtrCopy, ArgPtr);
+    int ret = std::vsnprintf(pBuffer, currentBufferSize, FormatString, ArgPtrCopy);
+    va_end(ArgPtrCopy);
     if (ret < 0 || ((u32)ret >= (currentBufferSize - 1)))
     {
       currentBufferSize *= 2;

src/common/string_util.cpp

@@ -14,11 +14,15 @@ std::string StdStringFromFormat(const char* format, ...)
 
 std::string StdStringFromFormatV(const char* format, std::va_list ap)
 {
+  std::va_list ap_copy;
+  va_copy(ap_copy, ap);
+
 #ifdef WIN32
-  int len = _vscprintf(format, ap);
+  int len = _vscprintf(format, ap_copy);
 #else
-  int len = std::vsnprintf(nullptr, 0, format, ap);
+  int len = std::vsnprintf(nullptr, 0, format, ap_copy);
 #endif
+  va_end(ap_copy);
 
   std::string ret;
   ret.resize(len);