Limit snappy input stream (#1738)

## Issue Addressed

N/A

## Proposed Changes

This PR limits the length of the stream received by the snappy decoder to be the maximum allowed size for the received rpc message type. Also adds further checks to ensure that the length specified in the rpc [encoding-dependent header](https://github.com/ethereum/eth2.0-specs/blob/dev/specs/phase0/p2p-interface.md#encoding-strategies) is within the bounds for the rpc message type being decoded.

beacon_node/eth2_libp2p/src/rpc/methods.rs

@@ -16,7 +16,8 @@ pub type MaxRequestBlocks = U1024;
 pub const MAX_REQUEST_BLOCKS: u64 = 1024;
 
 /// Maximum length of error message.
-type MaxErrorLen = U256;
+pub type MaxErrorLen = U256;
+pub const MAX_ERROR_LEN: u64 = 256;
 
 /// Wrapper over SSZ List to represent error message in rpc responses.
 #[derive(Debug, Clone)]
@@ -256,7 +257,7 @@ pub enum RPCCodedResponse<T: EthSpec> {
 }
 
 /// The code assigned to an erroneous `RPCResponse`.
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum RPCResponseErrorCode {
     RateLimited,
     InvalidRequest,