restore state event is passed an uninitialized ilist field for non-fcache cases

instrument_restore_nonfcache_state_prealloc() creates a dr_restore_state_info_t but leaves its fragment_info.ilist field unintialized. I hit this in #5686 where my new bbdup restore event was crashing walking this bogus instruction list.

Could the uninit ilist explain any unknown filed crashes? Looks like drreg already has a check for cache_start_pc being NULL and it bails on both sides of its ilist==NULL check so it happens to handle any invalid ilist value.