Fix for wrong size of message.length

[rbarreiros]

Initialize message.length on first packet (don't leave it at 0) and fix for 2 packet message sizes. I noticed that the end of message is already covered with setting message length with the mPendingMessageExpectedLength as it should, and that there's a check if the message is realtime, as send() in master branch calls getStatus() which screws up realtime midi messages (0xf8).