fix a rare autobug


0java.nio.BufferUnderflowException
1at java.nio.ByteArrayBuffer.getShort(ByteArrayBuffer.java:226)
2at org.mavlink.messages.ardupilotmega.msg_gps_raw_int.decode(msg_gps_raw_int.java:78)
3at org.mavlink.messages.MAVLinkMessageFactory.getMessage(MAVLinkMessageFactory.java:330)
4at org.mavlink.MAVLinkReader.readNextMessage(MAVLinkReader.java:212)
5at org.mavlink.MAVLinkReader.getNextMessage(MAVLinkReader.java:130)
6at com.geeksville.mavlink.MavlinkStreamReceiver$$anonfun$com$geeksville$mavlink$MavlinkStreamReceiver$$rxWorker$1.apply(MavlinkStreamReceiver.scala:130)
7at com.geeksville.mavlink.MavlinkStreamReceiver$$anonfun$com$geeksville$mavlink$MavlinkStreamReceiver$$rxWorker$1.apply(MavlinkStreamReceiver.scala:92)
8at com.geeksville.util.Using$.using(Using.scala:20)
9at com.geeksville.mavlink.MavlinkStreamReceiver.com$geeksville$mavlink$MavlinkStreamReceiver$$rxWorker(MavlinkStreamReceiver.scala:92)
10at com.geeksville.mavlink.MavlinkStreamReceiver$$anonfun$1.apply(MavlinkStreamReceiver.scala:48)
11at com.geeksville.mavlink.MavlinkStreamReceiver$$anonfun$1.apply(MavlinkStreamReceiver.scala:48)
12at com.geeksville.util.ThreadTools$$anon$1.run(ThreadTools.scala:11)
13at java.lang.Thread.run(Thread.java:841)
14java.nio.BufferUnderflowException
15at java.nio.ByteArrayBuffer.getShort(ByteArrayBuffer.java:226)
16at org.mavlink.messages.ardupilotmega.msg_gps_raw_int.decode(msg_gps_raw_int.java:78)
17at org.mavlink.messages.MAVLinkMessageFactory.getMessage(MAVLinkMessageFactory.java:330)
18at org.mavlink.MAVLinkReader.readNextMessage(MAVLinkReader.java:212)
19at org.mavlink.MAVLinkReader.getNextMessage(MAVLinkReader.java:130)