Fix BytesReader.read offset handling

[peter-jerry-ye]

BytesReader.read checks offset >= self.len and wraps offsets with % bytes.length(). The offset should be validated against the destination buffer length, and modulo should not be used (it can corrupt writes).

Suggested fix: compare offset to bytes.length() and return None for out-of-range offsets. Remove modulo wrapping.

Refs: lib/reader_impl.mbt.