Increment `self.index` before calling `Iterator::self.a.__iterator_get_unchecked...

Increment `self.index` before calling `Iterator::self.a.__iterator_get_unchecked` in `Zip` `TrustedRandomAccess` specialization

Otherwise if `Iterator::self.a.__iterator_get_unchecked` panics the
index would not have been incremented yet and another call to
`Iterator::next` would read from the same index again, which is not
allowed according to the API contract of `TrustedRandomAccess` for
`!Clone`.

Fixes https://github.com/rust-lang/rust/issues/81740