SUSPEND/GC bug: OPSYFT

[Jonathan A. Rees <JAR at MIT-MC>]

The location OPSYFT in list space is clobberred by the startup code
that gets run after a suspend happens.  However, that list cell is
subject to garbage collection; if it can 
get on the freelist (it can; don't ask me how - all I
know is that in Lisp 2088, on a Tops-20 at Yale, it has happened), the cell can get allocated for another use
before the SUSPEND, and clobberred after.
In our case it was the value word for a SUBR property in a PLIST;
the symptom was UNBOUND PC WITH ILLEGAL OPCODE or something of
that nature.
Look at the code near TNXSET and the comments before and after TNXSET.