RLP list decoding too lax for objects

[kdeme]

There seems to be no list size check done when decoding an object (possible also in general for lists?).

e.g. when decoding ef8c000000000000000000000000a0000000000000000000000000000000000000000000000000000000000000000080 to a WhoAreYouObj, that works as expected, as the rlp is valid.

However, decoding c08c000000000000000000000000a0000000000000000000000000000000000000000000000000000000000000000080 to a WhoAreYouObj, also works, while clearly the list size (c0 -> size 0) is not correct.

Only spotted this by adding a re-encode step and comparing with original payload in a fuzzing test.

Perhaps related with #254

[mjfh]

Just ran into that issue when debugging wire protocol datagrams as processed by checkedRlpRead(). In particular, r.read(MsgType) succeeds when r is a non-empty list and MsgType is EmptyList and also when r is a list of more than one element and MsgType is DisconnectionReasonList, a single element object. The latter one coincides with the reported issue, above.