Fix a bug which could cause imported events to set themselves as their own parents

Summary: Ref T11808. This variable is wrong, and would sometimes cause events to set themsevles as their own parents. They would then fail to load, and disrupt cursor paging.

Test Plan:

• Reproduced T11808 locally by reloading test data 2+ times, creating events with themselves as their own parents.
• Appplied fix.
• Nuked data, reloaded, no more self-parents.
• Test datafile:
  T11808.ics5 KBDownload

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11808

Differential Revision: https://secure.phabricator.com/D16793