player/Thread: don't restart unseekable song after failed seek attempt

The check IsSeekableCurrentSong() was added by commit
44b200240f in version 0.20.19, but it
caused a regression: by doing the branch only if the current song is
seekable, the player would restart the current song if it was not
seekable, and later the initial seek would fail; but we already know
it's not seekable, and so we should fail early.

src/player/Thread.cxx

@@ -599,6 +599,19 @@ Player::SeekDecoder() noexcept
 {
 	assert(pc.next_song != nullptr);
 
+	if (pc.seek_time > SongTime::zero() && // TODO: allow this only if the song duration is known
+	    dc.IsUnseekableCurrentSong(*pc.next_song)) {
+		/* seeking into the current song; but we already know
+		   it's not seekable, so let's fail early */
+		/* note the seek_time>0 check: if seeking to the
+		   beginning, we can simply restart the decoder */
+		pc.next_song.reset();
+		pc.SetError(PlayerError::DECODER,
+			    std::make_exception_ptr(std::runtime_error("Not seekable")));
+		pc.CommandFinished();
+		return true;
+	}
+
 	CancelPendingSeek();
 
 	{