snapshot: check snapshot file size
snapshot: check snapshot file size

When trying to load a snapshot shorter than `size_of::<u64>()`, the
snapshot load function would subtract with overflow and cause
Firecracker to panic. This commit fixes this problem by catching the
overflow.

Signed-off-by: George Pisaltu <084beef259e7feee20f091a1c4e2b24ee6aa34bb@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker