fixed bug in pl_add_meta which was skipping some of the fields, especially when using pl_items_copy_junk
added sanity check into pl_items_copy_junk, to make sure we won't dereference null pointers
lastfm: send data for tracks with unknown duration, with playtime calculated since last trackchange event
vfs_curl: send trackchange event on metadata changes, mainly so that lastfm plugin can scrobble tracks from radio