decoder_thread: eliminate jitter after seek failure

Don't clear the music pipe when seeking has failed - check the
"seeking" flag instead of "command==SEEK".  Clear the "seeking" flag
in decoder_seek_error().

NEWS

@@ -8,6 +8,7 @@ ver 0.15.7 (2009/??/??)
 * tags:
   - id3: fix ID3v1 charset conversion
 * decoders:
+  - eliminate jitter after seek failure
   - ffmpeg: don't try to force stereo
   - wavpack: allow fine-grained seeking
 * mixer: explicitly close all mixers on shutdown

src/decoder_api.c

@@ -93,7 +93,9 @@ void decoder_command_finished(G_GNUC_UNUSED struct decoder * decoder)
 	       dc.seek_error || decoder->seeking);
 	assert(dc.pipe != NULL);
 
-	if (dc.command == DECODE_COMMAND_SEEK) {
+	if (decoder->seeking) {
+		decoder->seeking = false;
+
 		/* delete frames from the old song position */
 
 		if (decoder->chunk != NULL) {
@@ -124,6 +126,8 @@ void decoder_seek_error(struct decoder * decoder)
 	assert(dc.pipe != NULL);
 
 	dc.seek_error = true;
+	decoder->seeking = false;
+
 	decoder_command_finished(decoder);
 }