fix pointer invalidation when extnding a vector from an untrusted iterator
fix pointer invalidation when extnding a vector from an untrusted iterator

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