Pike.identify_cycle: Fixed global variable overwrite.

Pike.identify_cycle() uses a mapping internally, and insert into
mappings can call Pike code (via LFUN:__hash()), which could
have a different thread also start messing with the same variables.

This fixes the multi-thread case, but there's still a problem if it
starts to recurse (ie LFUN::__hash() calling Pike.identify_cycle()
or Pike_count_memory()).