server: Avoid accessing free'd thread pointers.

[Mike McCormack]

Eric Pouech wrote:
> IMO this should be fixed by rewriting correctly the _SAFE version of the 
> list macros, which are to blame here.
> I have this somewhere, I'll send it later on.

The only way to prevent the next pointer (or any other pointer in the 
thread list) from being free'd is to grab it.   Since the LIST macros 
doesn't understand pointer grabbing, I don't see a way to fix it by 
changing the macro...

Mike