Avoid out of bounds access in arpack neupd_wrapper

Trying to avoid out of bounds access in eupd wrapper in #7725 

This should really not be happening, since these eigenvalues are in complex conjugate pairs and occur together.

Cc: @jutho

`zero(T)` is not necessary for comparison. Every numeric type should be able to give the correct answer for `x == 0`; it's just a boolean result.