spl: fit: Skip attempting to load 0 length image

When, for various reasons, a bad FIT image is used where a loadable
image is marked as 0 length, attempt is made for a 0 length allocation and
read of 0 byte read operation.

Instead provide warning in log and skip attempting to do such a load.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Aswath Govindraju <a-govindraju@ti.com>