buffer: fix crash for invalid index types

2555cb4a introduced a crash
when a non-number value was passed to `ParseArrayIndex()`.

We do not always have JS typechecking for that in place, though.
This returns back to the previous behavior of coercing values
to integers, which is certainly questionable.

Refs: https://github.com/nodejs/node/pull/22129
Fixes: https://github.com/nodejs/node/issues/23668

PR-URL: https://github.com/nodejs/node/pull/25154


Reviewed-By: James M Snell <jasnell@gmail.com>