Fix NPE when there is no audio capture device

src/net/java/sip/communicator/impl/neomedia/GatherEntropy.java

@@ -145,8 +145,13 @@ private class GatherAudio extends Thread implements BufferTransferHandler
          */
         private boolean prepareAudioEntropy()
         {
+            CaptureDeviceInfo audioCaptureDevice = 
+                    deviceConfiguration.getAudioCaptureDevice();
+            if (audioCaptureDevice == null)
+                return false;
+
             MediaLocator audioCaptureDeviceLocator
-                = deviceConfiguration.getAudioCaptureDevice().getLocator();
+                = audioCaptureDevice.getLocator();
 
             if (audioCaptureDeviceLocator == null)
                 return false;