Not sure why, but opening a non-exisiting file is not returning
an error. Well, let's add another check to cover troubles, by
identifying if no lines were filled at the cache array.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>