fix uninitialized values in get_audio_fileinfo()

[geuma]

for instance

is
$$info{GENRE} = $tag->{'GENRE'} if defined($tag->{'GENRE'}) && length($tag->{'GENRE'}) > 0;
a better idea than
$$info{GENRE} = $tag->{'GENRE'} if length($tag->{'GENRE'}) > 0;