PR17249: tolerate early module notifier calls with null mod->sect_attrs

PR17249: tolerate early module notifier calls with null mod->sect_attrs

In the case of MODULE_STATE_COMING, we may encounter NULL sect_attrs,
and we must not crash.  Sadly, that case can mean the loss of ability
to probe module-init functions - i.e., breaking the bz6503 test case.

* runtime/transport/symbols.c (_stp_module_notifier): Don't assume
  that mod->sect_attrs is valid.  Treat COMING|LIVE notifications
  similarly, except LIVE should assume init.* gone gone gone,
  she been gone so long, she been gone gone gone so long.