Fix infinite loop in FTKSim caused by bad particle parent record
This fixes https://its.cern.ch/jira/browse/ATR-19145
In some older Sherpa samples, the truth record is mangled in some situations and truth particles' parents may not be fully stored. The affected loop was infinite because it would never reach the end of the iterator (which was null). We simply abort on recognizing the problem now.
This only affects the truth record of some Sherpa (< 2.2.1) samples, so it should be harmless for all tests and physics.