Reject overlong varints to avoid stream desync

[peter-jerry-ye]

In read_varint32/read_varint64 (and async variants), the decoder returns after 5/10 bytes even if the continuation bit is still set. This accepts overlong varints and leaves extra continuation bytes in the stream, which then desyncs subsequent field decoding.

Suggested fix: detect overlong varints and raise an error if the continuation bit is still set after the max length. Also mirror the change in async paths.

Refs: lib/reader.mbt, lib/async_reader.mbt.