expanding unit test for QAudioDecoder

Looking for feedback concerning:
implementation of the change
desired functionality of QAudioDecoder

Changed the behaviour of QAudioDecoder for
more sane error output

Change-Id: I82193a94b6fe1ef4202a4ac7bd95c607e0bee9c6
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

tests/auto/unit/qmultimedia_common/mockaudiodecoderservice.h

@@ -55,6 +55,7 @@ public:
         : QMediaService(parent)
     {
         mockControl = new MockAudioDecoderControl(this);
+        validControl = mockControl;
     }
 
     ~MockAudioDecoderService()
@@ -74,7 +75,18 @@ public:
         Q_UNUSED(control);
     }
 
+    void setControlNull()
+    {
+        mockControl = 0;
+    }
+
+    void setControlValid()
+    {
+        mockControl = validControl;
+    }
+
     MockAudioDecoderControl *mockControl;
+    MockAudioDecoderControl *validControl;
 };