Fix for #647.

In the logplayer, the data stream is read ahead for messages, which is used to determine the size of the output bytearrays. Previously, this wasn't done correctly resulting in non-ideal behavior. Now, the next message is scanned properly, so the only bytearrays that are emitted from QGCMAVLinkLogPlayer are MAVLink messages in their entirety. This also has the benefit of skipping invalid/corrupted messages in the log that's being played back.