Blender Git Loki

Fix T49283: Crash in BKE_ptcache_make_particle_key.

This is really hack-fix actually, not sure why `get_pointcache_keys_for_time()` seems to assume
it will always find key for given part index at least for current frame, and whether this assumption
is wrong or whether bug happens elsewhere...

Anyway, this is to be wiped out in 2.8, so no point loosing too much time on it, for now merely
returning unchanged (i.e. zero'ed) ParticleKeys in case index2 is invalid. Won't hurt anyway,
even if this did not crash in release builds, would be returning giberish values.