host-ctr: fix references for non-ECR images

[etungsten]

Discovered this bug when I was working on #433.

Issue #, if available: n/a

Description of changes: Fixes a bug introduced in #382 where ref isn't set when pulling images from non-ECR registries

Testing:
Before the fix:

$ sudo ./host-ctr -source registry.hub.docker.com/library/busybox:latest -pull-image-only -containerd-socket /run/containerd/containerd.sock
ERRO[0000] Failed to pull ctr image: failed to resolve reference "": hostname required  ref=

Now can pull images from docker hub, etc and not just ECR:

$ sudo ./host-ctr -source registry.hub.docker.com/library/busybox:latest -pull-image-only -containerd-socket /run/containerd/containerd.sock
INFO[0000] Pulled successfully                           img="registry.hub.docker.com/library/busybox:latest"
INFO[0000] Unpacking...                                  img="registry.hub.docker.com/library/busybox:latest"

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