Catch malformed events throwing IndexError.
Catch malformed events throwing IndexError.

Manifests as:

Traceback (most recent call last):
  File ./bin/backfill-emailed-events, line 30, in <module>
    import_attached_events(db_session, account, message)
  File /usr/share/python/inbox-sync/local/lib/python2.7/site-packages/inbox/events/ical.py, line 216, in import_attached_events
    part.block.data)
  File /usr/share/python/inbox-sync/local/lib/python2.7/site-packages/inbox/events/ical.py, line 25, in events_from_ics
    cal = iCalendar.from_ical(ics_str)
  File /usr/share/python/inbox-sync/local/lib/python2.7/site-packages/icalendar/cal.py, line 343, in from_ical
    component = stack[-1]
IndexError: list index out of range

gale320/sync-engine,nylas/sync-engine,PriviPK/privipk-sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,closeio/nylas,EthanBlackburn/sync-engine,Eagles2F/sync-engine,EthanBlackburn/sync-engine,closeio/nylas,EthanBlackburn/sync-engine,wakermahmud/sync-engine,PriviPK/privipk-sync-engine,nylas/sync-engine,jobscore/sync-engine,nylas/sync-engine,jobscore/sync-engine,closeio/nylas,PriviPK/privipk-sync-engine,jobscore/sync-engine,ErinCall/sync-engine,nylas/sync-engine,ErinCall/sync-engine,ErinCall/sync-engine,EthanBlackburn/sync-engine,gale320/sync-engine,closeio/nylas,PriviPK/privipk-sync-engine,gale320/sync-engine,wakermahmud/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,EthanBlackburn/sync-engine,gale320/sync-engine,wakermahmud/sync-engine,PriviPK/privipk-sync-engine,Eagles2F/sync-engine