Correct ChaCha[Dual]PolyReadAdapter extra stream contents logic

`ChaCha[Dual]PolyReadAdapter` currently read the encrypted object
using `Readable` through the ChaCha stream (including the Poly1305
HMAC), but then consume any remaining bytes directly. This results
in any extra bytes not consumed by the desired type's `Readable`
being ignored and not included in the HMAC check.

This is likely not the desired behavior - if we get some data which
has extra slack at the end we ignore, it should still be
authenticated as the sender likely thinks that data has meaning and
included it in their HMAC check.

Luckily, I believe this is currently dead code -
`ChaCha[Dual]PolyReadAdapter` are only used for TLV stream reads
which consume the full underlying stream. However, if either is
used for non-TLV-streams in the future, this may be important.

Here we simply push any extra bytes read through the
ChaCha20Poly1305 reader, ensuring extra data is included in the
HMAC check.

Author: TheBlueMatt

lightning/src/crypto/streams.rs

@@ -128,7 +128,10 @@ impl<T: Readable> LengthReadableArgs<([u8; 32], [u8; 32])> for ChaChaDualPolyRea
 			ChaChaDualPolyReader { chacha: &mut chacha, poly: &mut mac, read_len: 0, read: s };
 
 		let readable: T = Readable::read(&mut chacha_stream)?;
-		chacha_stream.read.eat_remaining()?;
+		while chacha_stream.read.bytes_remain() {
+			let mut buf = [0; 256];
+			chacha_stream.read(&mut buf)?;
+		}
 
 		let read_len = chacha_stream.read_len;
 
@@ -203,7 +206,10 @@ impl<T: Readable> LengthReadableArgs<[u8; 32]> for ChaChaPolyReadAdapter<T> {
 		let s = FixedLengthReader::new(r, decrypted_len);
 		let mut chacha_stream = ChaChaPolyReader { chacha: &mut chacha, read: s };
 		let readable: T = Readable::read(&mut chacha_stream)?;
-		chacha_stream.read.eat_remaining()?;
+		while chacha_stream.read.bytes_remain() {
+			let mut buf = [0; 256];
+			chacha_stream.read(&mut buf)?;
+		}
 
 		let mut tag = [0 as u8; 16];
 		r.read_exact(&mut tag)?;