fix: apply message size limit before decoding message

If we apply the message size limit after decoding the message it's
too late as we've already processed the bad message.

Instead, if the buffer full of unprocessed messages grows to be
large than the max message size (e.g. we have not recieved a complete
message under the size limit), throw an error which will cause the
stream to be reset.

Author: achingbrain

src/decode.ts

@@ -3,7 +3,7 @@ import { Uint8ArrayList } from 'uint8arraylist'
 import type { Source } from 'it-stream-types'
 import type { Message } from './message-types.js'
 
-export const MAX_MSG_SIZE = Math.pow(2, 20)
+export const MAX_MSG_SIZE = 1 << 20 // 1MB
 
 interface MessageHeader {
   id: number