safe_io: do not set ending \0 in safe_read_file()

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Merge pull request #3018 from tchaikov/wip-fix-safe_read_file

safe_io: do not set ending \0 in safe_read_file()

Reviewed-by: Samuel Just <sjust@redhat.com>
Reviewed-by: Loic Dachary <ldachary@redhat.com>