use WIN_StringToUTF8W macro instead of WIN_StringToUTF8, where needed:

i.e. where the string is known guaranteed to be WCHAR*, in:
- SDL_dinputjoystick.c (WIN_IsXInputDevice): VARIANT->var is BSTR (WCHAR*)
- SDL_rawinputjoystick.c (RAWINPUT_AddDevice): string is WCHAR*
- SDL_windows_gaming_input.c (IEventHandler_CRawGameControllerVtbl_InvokeAdded):
  string is WCHAR*

There should be more of these..

src/joystick/windows/SDL_dinputjoystick.c

@@ -315,7 +315,7 @@ WIN_IsXInputDevice(const TCHAR *name, const GUID* pGuidProductFromDirectInput)
                 // Check if the device ID contains "IG_".  If it does, then it's an XInput device
                 // This information can not be found from DirectInput 
                 if (SDL_wcsstr(var.bstrVal, L"IG_")) {
-                    char *bstrVal = WIN_StringToUTF8(var.bstrVal);
+                    char *bstrVal = WIN_StringToUTF8W(var.bstrVal);
 
                     // If it does, then get the VID/PID from var.bstrVal
                     DWORD dwPid = 0, dwVid = 0, dwVidPid;