decoder/Bridge: call UpdateStreamTag() only if there is no pending seek

If UpdateStreamTag() gets called while an initial seek is pending, the
result will never be submitted to a MusicChunk.  By avoiding the
UpdateStreamTag() call in that case (by moving UpdateStreamTag() to
after the PrepareInitialSeek() check), the song_tag is preserved until
UpdateStreamTag() is called again from SubmitData().

This fixes missing tags in the "httpd" output.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1137

src/decoder/Bridge.cxx

@@ -581,10 +581,6 @@ DecoderBridge::SubmitTag(InputStream *is, Tag &&tag) noexcept
 
 	decoder_tag = std::make_unique<Tag>(std::move(tag));
 
-	/* check for a new stream tag */
-
-	UpdateStreamTag(is);
-
 	/* check if we're seeking */
 
 	if (PrepareInitialSeek())
@@ -593,6 +589,10 @@ DecoderBridge::SubmitTag(InputStream *is, Tag &&tag) noexcept
 		   function here */
 		return DecoderCommand::SEEK;
 
+	/* check for a new stream tag */
+
+	UpdateStreamTag(is);
+
 	/* send tag to music pipe */
 
 	if (stream_tag != nullptr)