i2p: limit the size of incoming messages

Put a limit on the amount of data `Sock::RecvUntilTerminator()` can read
if no terminator is received.

In the case of I2P this avoids a runaway (or malicious) I2P proxy
sending us tons of data without a terminator before a timeout is
triggered.

src/i2p.h

@@ -40,6 +40,14 @@ struct Connection {
 
 namespace sam {
 
+/**
+ * The maximum size of an incoming message from the I2P SAM proxy (in bytes).
+ * Used to avoid a runaway proxy from sending us an "unlimited" amount of data without a terminator.
+ * The longest known message is ~1400 bytes, so this is high enough not to be triggered during
+ * normal operation, yet low enough to avoid a malicious proxy from filling our memory.
+ */
+static constexpr size_t MAX_MSG_SIZE{65536};
+
 /**
  * I2P SAM session.
  */