mpdthumb: Fix failure when readpicture/albumart both return data.

mpdthumb: Fix failure when readpicture/albumart both return data.

If a file has both embedded cover art and cover art in its directory,
both commands will return data and mpdthumb prints two lines instead
of one as expected.  Fix this by skipping the check for albumart data
if valid readpicture data was returned.