Add checks to packed event parser to avoid index out of range errors / undefined behavior

[theAnuragMishra]

prior to this there were no checks of length before accessing indices. this could be a reason for ub.

[hoytech]

Packed events are always supposed to be trusted data, so in theory there's no need for these checks. However, since the impact is so minimal, maybe you're right to have these checks in as defensive code. Thanks!