Hopefully fix #1910 'When I use the command "mrl playlist_add vlc_play", "playlist_ad...

Hopefully fix #1910 'When I use the command "mrl playlist_add vlc_play", "playlist_add" used to return the index of the item added to the playlist wheras in the last nightly build, "playlist_add" just add 0 to the stack. Thus, "vlc_play" just play the first element of the playlist in spite of playing the desired file.' ... and fix an unrelated compilation warning which ended up being a bit more complex to fix than I though.