Memory corruption via repeated unchecked Zip access

Memory-safety flaw in the Rust standard library before 1.52.0 allows attackers to create conflicting mutable references through Zip. In Zip::next(), the TrustedRandomAccess specialization increments self.index only after the first unchecked iterator access, so a panic leaves the index unchanged and a later next() reads that index again. Triggering the flaw requires an underlying iterator to panic and the Zip iterator to be used again; the resulting contract violation creates two mutable references to the same memory and causes undefined behavior.