Fix a segfault when using "Shift+a" with dummy audio output.

This segfault happend because :
 * the variable "audio-device" wasn't created
 * hotkeys module (and others) expect the variable to exist.
(cherry picked from commit 94ca05ff)
Signed-off-by: Rémi Duraffort <ivoire@videolan.org>

modules/misc/dummy/aout.c

@@ -62,7 +62,11 @@ int OpenAudio ( vlc_object_t * p_this )
     {
         p_aout->output.i_nb_samples = FRAME_SIZE;
     }
-    return 0;
+
+    /* Create the variable for the audio-device */
+    var_Create( p_aout, "audio-device", VLC_VAR_INTEGER | VLC_VAR_HASCHOICE );
+
+    return VLC_SUCCESS;
 }
 
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