microbit: Added validation case in MicroBitRadioDatagram::recv()

Introduced missed validation case in MicroBitRadioDatagram::recv().
Added PacketBuffer::EmptyPacket as a NULL reference.

inc/PacketBuffer.h

@@ -233,6 +233,8 @@ class PacketBuffer
      * @endcode
      */
     void setRSSI(uint8_t rssi);
+
+    static PacketBuffer EmptyPacket;
 };
 
 #endif

source/PacketBuffer.cpp

@@ -1,5 +1,8 @@
 #include "MicroBit.h"
 
+// Create the EmptyPacket reference.
+PacketBuffer PacketBuffer::EmptyPacket = PacketBuffer(1);
+
 /**
  * Default Constructor. 
  * Creates an empty Packet Buffer. 

source/ble-services/MicroBitRadioDatagram.cpp

@@ -57,6 +57,9 @@ int MicroBitRadioDatagram::recv(uint8_t *buf, int len)
  */
 PacketBuffer MicroBitRadioDatagram::recv()
 {
+    if (rxQueue == NULL)
+        return PacketBuffer::EmptyPacket;
+
     FrameBuffer *p = rxQueue;
     rxQueue = rxQueue->next;