unmarshalConvertedConfig: avoid using the updated image's ref
unmarshalConvertedConfig: avoid using the updated image's ref

The image returned by UpdatedImage() doesn't have a reference, so
attempting to include the result's name in an error message causes a
crash.  Assign it to a temporary variable first so that we can use the
name of the un-updated version in an error message.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #1960
Approved by: TomSweeneyRedHat

projectatomic/buildah,nalind/buildah-1,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,rhatdan/buildah,nalind/buildah,nalind/buildah-1,rhatdan/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah