mod_smacks: Fix traceback due to session being destroyed in send() Sending something can cause the OS to notice that the connection is dead and then the connection can be dead at this point. More likely if opportunistic_writes is enabled.

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.