plugins: Add plugin_release_audio_buffer().

Some plugins grab the whole audio buffer and still want to start playback
somehow (e.g. random_folder_advance_config). Since 22e802e the plugin buffer
is allocated via buflib and has to be released explicitely. For these plugins
the automatic free on exit is not sufficient and they need an API function for
that.

Fixes OOM panic on random_folder_advance_config when using start
shuffled playback.

Change-Id: I0d351daa782cb829f4ff80d34c05f40a2e0c142f

apps/plugins/random_folder_advance_config.c

@@ -541,6 +541,9 @@ static int start_shuffled_play(void)
         }
     }
     rb->splash(HZ, "Done");
+    /* the core needs the audio buffer back in order to start playback. */
+    list = NULL;
+    rb->plugin_release_audio_buffer();
     rb->playlist_start(0, 0, 0);
     return 1;
 }