Revert "cpio: Fix zero-length files"

This reverts commit 860d6c53e2ec0cec0e4f5411029cd62767059b6d.  It makes
cpiod.so completely ignore zero-length files.  This breaks 'make check'
because the tmp directory is no longer created, for example.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>