Fixed buffer overflow in message header

- Fixed buffer overflow by adding a reset method that  clears the buffer and sets the position to zero before re-writing the header
Tags: #story[1047]

src/main/java/seng302/server/messages/Header.java

@@ -37,16 +37,27 @@ public class Header {
     private void putInBuffer(byte[] bytes, long val){
         byte[] tmp = bytes.clone();
         Message.reverse(tmp);
-
+        
         buff.put(tmp);
         buffPos += tmp.length;
         buff.position(buffPos);
     }
 
+    /**
+     * Reset the buffer
+     */
+    public void reset(){
+        buffPos = 0;
+        buff.clear();
+        buff.position(buffPos);
+    }
+
     /**
      * @return a ByteBuffer containing the message header
      */
     public ByteBuffer getByteBuffer(){
+        reset();
+
         putInBuffer(ByteBuffer.allocate(1).put((byte)syncByte1).array(), syncByte1);
 
         putInBuffer(ByteBuffer.allocate(1).put((byte)syncByte2).array(), syncByte2);