Per tendermint spec, each Channel has a globally unique byte id, which
is mapped to uint8 in Go. However, the proto PacketMsg.ChannelID field
is declared as int32, and when receive the packet, we cast it to a byte
without checking for possible overflow. That leads to a malform packet
with invalid channel id is sent successfully.
To fix it, we just add a check for possible overflow, and return invalid
channel id error.
Fixed#6521