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/decoder/Control.hxx

@@ -320,6 +320,11 @@ public:
 	gcc_pure
 	bool IsCurrentSong(const DetachedSong &_song) const noexcept;
 
+	gcc_pure
+	bool IsUnseekableCurrentSong(const DetachedSong &_song) const noexcept {
+		return !seekable && IsCurrentSong(_song);
+	}
+
 	gcc_pure
 	bool IsSeekableCurrentSong(const DetachedSong &_song) const noexcept {
 		return seekable && IsCurrentSong(_song);