Don't generate a zero-length packet in case of errors.

The packet converter used to generate packets of length zero
when it encountered errors during conversion. This caused
exceptions in packet analyzers.

Now the converter returns null in case of error. Appropriate
checks have been added to the code that uses the return value.

tools/cooja/apps/mspsim/src/org/contikios/cooja/mspmote/interfaces/CC2420RadioPacketConverter.java

@@ -198,7 +198,7 @@ public class CC2420RadioPacketConverter {
     System.arraycopy(data, 6 /* skipping preamble+synch+len */, originalData, 0, originalLen);
     if (len < 0) {
       /*logger.warn("No cross-level conversion available: negative packet length");*/
-      return new ConvertedRadioPacket(new byte[0], originalData);
+      return null;
     }
     byte convertedData[] = new byte[len];
     System.arraycopy(data, pos, convertedData, 0, len);

tools/cooja/apps/mspsim/src/org/contikios/cooja/mspmote/interfaces/Msp802154Radio.java

@@ -99,6 +99,7 @@ public class Msp802154Radio extends Radio implements CustomDataRadio {
       public void receivedByte(byte data) {
         if (!isTransmitting()) {
           lastEvent = RadioEvent.TRANSMISSION_STARTED;
+          lastOutgoingPacket = null;
           isTransmitting = true;
           len = 0;
           expMpduLen = 0;
@@ -139,10 +140,12 @@ public class Msp802154Radio extends Radio implements CustomDataRadio {
 
         if (((expMpduLen & 0x80) == 0) && len == expMpduLen + 6 && isSynchronized) {
           lastOutgoingPacket = CC2420RadioPacketConverter.fromCC2420ToCooja(buffer);
+          if (lastOutgoingPacket != null) {
             lastEvent = RadioEvent.PACKET_TRANSMITTED;
             //logger.debug("----- 802.15.4 PACKET TRANSMITTED -----");
             setChanged();
             notifyObservers();
+          }
           isSynchronized = false;
         }
       }

tools/cooja/java/org/contikios/cooja/motes/AbstractApplicationMote.java

@@ -76,6 +76,7 @@ public abstract class AbstractApplicationMote extends AbstractWakeupMote impleme
         if (radio.getLastPacketReceived() != null)
             receivedPacket(radio.getLastPacketReceived());
       } else if (radio.getLastEvent() == Radio.RadioEvent.TRANSMISSION_FINISHED) {
+        if (radio.getLastPacketTransmitted() != null)
             sentPacket(radio.getLastPacketTransmitted());
       }
     }

tools/cooja/java/org/contikios/cooja/plugins/RadioLogger.java

@@ -517,10 +517,12 @@ public class RadioLogger extends VisPlugin {
           return;
         }
         final RadioConnectionLog loggedConn = new RadioConnectionLog();
+        loggedConn.packet = conn.getSource().getLastPacketTransmitted();
+        if (loggedConn.packet == null)
+          return;
         loggedConn.startTime = conn.getStartTime();
         loggedConn.endTime = simulation.getSimulationTime();
         loggedConn.connection = conn;
-        loggedConn.packet = conn.getSource().getLastPacketTransmitted();
         java.awt.EventQueue.invokeLater(new Runnable() {
           @Override
           public void run() {