output/control: always close the `AudioOutputSource` in `RELEASE`

Fixes a crash bug with `always_on` outputs which occurs because the
`AudioOutputSource` still has a pointer to an outdated `MusicChunk`.

Fixes #415

NEWS

@@ -6,7 +6,7 @@ ver 0.21.2 (not yet released)
   - ffmpeg: fix broken sound with certain codecs
 * output
   - alsa: fix high CPU usage with dmix
-  - httpd: fix two crash bugs
+  - httpd: fix three crash bugs
 * mixer
   - alsa: fix more rounding errors
 * fix zlib support

src/output/Thread.cxx

@@ -467,7 +467,12 @@ AudioOutputControl::Task() noexcept
 
 			if (always_on) {
 				/* in "always_on" mode, the output is
-				   paused instead of being closed */
+				   paused instead of being closed;
+				   however we need to close the
+				   AudioOutputSource because its data
+				   have been invalidated by stopping
+				   the actual playback */
+				source.Close();
 				InternalPause();
 			} else {
 				InternalClose(false);