Window can read out of bounds if Read instance returns more bytes than buffer size

Summary

`rdiff` performs a diff of two provided strings or files. As part of its reading code it uses the return value of a `Read` instance to set the length of its internal character vector. If the `Read` implementation claims that it has read more bytes than the length of the provided buffer, the length of the vector will be set to longer than its capacity. This causes `rdiff` APIs to return uninitialized memory in its API methods.