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

To handle all types, this should use `isnan` instead of `!==`. `===` only says NaNs are equal if they're the same type, and identical in every other way too.