Prevent Zip specialization from calling __iterator_get_unchecked twice with the same index after calling next_back
Prevent Zip specialization from calling __iterator_get_unchecked twice with the same index after calling next_back

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust