mpvcore/playlist.c: Avoid NULL reference in playlist_add_base_path()

Before this patch, those will cause a crash:

  mpv -playlist /dev/null
  mpv -playlist <(bla) # if the result of bla is empty

Signed-off-by: Mohammad Alsaleh <CE.Mohammad.AlSaleh@gmail.com>
Signed-off-by: Stefano Pigozzi <stefano.pigozzi@gmail.com>