NTLM: gracefully handle null terminator in Target Name

I've ran into a bug when I tried to authenticate to a WinGate proxy server with NTLM authentication.

The server sends the domain name (Target Name) in the challenge with the null terminator included. This is not permitted according to the documentation (https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-dtyp/94a16bb6-c610-4cb9-8db6-26f15f560061).

Qt works correctly when it assumes that the string doesn't contain the null terminator when it converts it to QString for further usage.

However, to make it more resilient and to be able to authenticate with WinGate (and who knows what other servers have this issues), we can truncate the string at the null terminator.

I've attached a patch with the fix. I've renamed the function because it's only used for Target Name and because of this logic, it should not be used for other Unicode conversions. (Null characters are valid in a Unicode string, but not in a domain name).