tools/bp_gdb.py: stop intrusive::set iteration at null pointer
tools/bp_gdb.py: stop intrusive::set iteration at null pointer

This is a kludge; I don't know how intrusive::set really works
internally, but this kludge is good enough for debugging right now.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy