Ensure .mapper is set on _ColumnEntity

Ensure .mapper is set on _ColumnEntity

_ColumnEntity didn't seem to have .mapper present, which
due to the way _mapper_zero() worked didn't tend to come
across it.   With :ticket:`3608` _mapper_zero() has
been simplified so make sure this is now present.
Also ensure that _select_from_entity is an entity and
not a mapped class, though this does not seem to matter
at the moment.

Fixes: #3836
Change-Id: Id6dae8e700269b97de3b01562edee95ac1e01f80