Logs are saved constantly. A crash should not cause you to lose any logs unless for whatever reason the log file got corrupted by the crash, which isn't very likely. And if that did happen, you'd still have the file, but it would probably not allow writing to it or opening it.
If you aren't getting your logs saved constantly, then check your log settings in mIRC and make sure you do not have a script installed that changes how logs are saved.
