fix MultiByte <-> WideChar conversion return value on linux

MultiByteToWideChar/WideCharToMultiByte are supposed to include/exclude null termination
based on whether the source string buffer is null terminated.

This was done by comparing the input byte/wide character count to the output byte/wide
character count excluding null termination. This only works correctly if the input only
consists of ASCII characters. (ie. only characters that are single byte in UTF-8)

This commit changes the code to explicitly check for a null terminator in the input
and add one to the output size if the input was terminated.

lib/DxcSupport/Unicode.cpp

@@ -54,7 +54,8 @@ int MultiByteToWideChar(uint32_t /*CodePage*/, uint32_t /*dwFlags*/,
   size_t rv;
   const char *prevLocale = setlocale(LC_ALL, nullptr);
   setlocale(LC_ALL, "en_US.UTF-8");
-  if (lpMultiByteStr[cbMultiByte - 1] != '\0') {
+  const bool bIsNullTerminated = lpMultiByteStr[cbMultiByte - 1] == '\0';
+  if (!bIsNullTerminated) {
     char *srcStr = (char *)malloc((cbMultiByte + 1) * sizeof(char));
     strncpy(srcStr, lpMultiByteStr, cbMultiByte);
     srcStr[cbMultiByte] = '\0';
@@ -67,9 +68,9 @@ int MultiByteToWideChar(uint32_t /*CodePage*/, uint32_t /*dwFlags*/,
   if (prevLocale)
     setlocale(LC_ALL, prevLocale);
 
-  if (rv == (size_t)cbMultiByte)
-    return rv;
-  return rv + 1; // mbstowcs excludes the terminating character
+  if (bIsNullTerminated)
+    return rv + 1; // mbstowcs excludes the terminating character
+  return rv;
 }
 
 // WideCharToMultiByte is a Windows-specific method.
@@ -110,7 +111,8 @@ int WideCharToMultiByte(uint32_t /*CodePage*/, uint32_t /*dwFlags*/,
   size_t rv;
   const char *prevLocale = setlocale(LC_ALL, nullptr);
   setlocale(LC_ALL, "en_US.UTF-8");
-  if (lpWideCharStr[cchWideChar - 1] != L'\0') {
+  const bool bIsNullTerminated = lpWideCharStr[cchWideChar - 1] == L'\0';
+  if (!bIsNullTerminated) {
     wchar_t *srcStr = (wchar_t *)malloc((cchWideChar + 1) * sizeof(wchar_t));
     wcsncpy(srcStr, lpWideCharStr, cchWideChar);
     srcStr[cchWideChar] = L'\0';
@@ -123,9 +125,9 @@ int WideCharToMultiByte(uint32_t /*CodePage*/, uint32_t /*dwFlags*/,
   if (prevLocale)
     setlocale(LC_ALL, prevLocale);
 
-  if (rv == (size_t)cchWideChar)
-    return rv;
-  return rv + 1; // mbstowcs excludes the terminating character
+  if (bIsNullTerminated)
+    return rv + 1; // mbstowcs excludes the terminating character
+  return rv;
 }
 #endif // _WIN32