util/disk_cache: correctly use stat(3)

I forgot to error check stat() and also I wasn't using the subdir in
is_two_character_sub_directory().

Fixes: d7b3707c612 "util/disk_cache: use stat() to check if entry is a directory"
Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed