A followup of #816 found by fuzzing, but otherwise is obvious if we approach the functions from an outside attacker's perspective.

In PEXReactor.Receive, if a client passes in nil/no bytes, we deference the bytes in
https://github.com/tendermint/tendermint/blob/master/p2p/pex_reactor.go#L332

We should check that the bytes are not empty lest we get a crash