Handle encoding failures in Windows thread names

Internally at AdaCore, we noticed that the new Windows thread name
code could fail.  First, it might return a zero-length string, but in
gdb conventions it should return nullptr instead.  Second, an encoding
failure could wind up showing replacement characters to the user; this
is confusing and not useful; it's better to recognize such errors and
simply discard the name.  This patch makes both of these changes.