When looking up the postprocessor function from shadow files, id::find_func()
would abort() if the function wasn't available instead of falling back
to the default postprocessor.
Fix by using id::find() and checking the type explicitly and also adding a
strict type check while at it.
This issue was tickled by loading the json-streaming-logs package,
Zeek creating shadow files containing its custom postprocessor function,
then restarting Zeek without the package loaded.
Closes#4562
