[initrd] Allow for zero-length initrd files

[initrd] Allow for zero-length initrd files

A zero-length initrd file will currently cause an endless loop during
reshuffling as the empty image is repeatedly swapped with itself.

Fix by terminating the inner loop before considering an image as a
candidate to be swapped with itself.

Reported-by: Pico Mitchell <pico@randomapplications.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>