demux_cue: fix audio file lookup inside subdirectory

The FILE field in a CUE sheet may contain a relative path with
subdirectories. It's not clear if this usecase is intended to be
supported, as none of the usual CD/DVD ripping tool generates this type
of FILE field in the CUE sheet. The specification is old and abandoned,
so this will likely never be clarified and it doesn't cost much to
support it so might as well...