output/Source: reset current_chunk in Open()

If the output is already open, the `current_chunk` pointer may be
bogus and out of sync with `SharedPipeConsumer::chunk`, leading to an
assertion failure in `SharedPipeConsumer::Consume()`.

Fixes #411

NEWS

@@ -2,6 +2,8 @@ ver 0.21.2 (not yet released)
 * decoder
   - ffmpeg: require FFmpeg 3.1 or later
   - ffmpeg: fix broken sound with certain codecs
+* output
+  - httpd: fix crash with "always_on"
 
 ver 0.21.1 (2018/11/04)
 * protocol

src/output/Source.cxx

@@ -41,8 +41,10 @@ AudioOutputSource::Open(const AudioFormat audio_format, const MusicPipe &_pipe,
 {
 	assert(audio_format.IsValid());
 
-	if (!IsOpen() || &_pipe != &pipe.GetPipe())
+	if (!IsOpen() || &_pipe != &pipe.GetPipe()) {
+		current_chunk = nullptr;
 		pipe.Init(_pipe);
+	}
 
 	/* (re)open the filter */