docker/image_dest: do not deref possibly-nil results
docker/image_dest: do not deref possibly-nil results

This avoids dereferencing a nil result when an error is
encountered on PATCH request.

Signed-off-by: Luca Bruno <54b63133bb17564df145c2c274f1bc31eb9aa743@coreos.com>

containers/image,containers/image,masters-of-cats/image