FIX: prevent jinja expert bot crash on logrotate

[monoidic]

This PR prevents this error on logrotate, which appears to be caused by init getting rerun after self.fields has already been updated by init?

Jinja-Expert - ERROR - Bot initialization failed.
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/intelmq/lib/bot.py", line 241, in __init__
    self.init()
  File "/usr/lib/python3/dist-packages/intelmq/bots/experts/jinja/expert.py", line 38, in init
    if not template.startswith("file:///"):
AttributeError: 'Template' object has no attribute 'startswith'

[kamil-certat]

Thanks for the change, LGTM!

which appears to be caused by init getting rerun after self.fields has already been updated by init?

This is typical situation on reloading the bot, when the process isn't stopped (as during the restart), and init() is expected to refresh the configuration

[sebix]

Merged, based on kamil's review.