db-ctl-base: Fix reference-following feature in get_row_by_id().

db-ctl-base: Fix reference-following feature in get_row_by_id().

If a particular column is supposed to be reached by following a reference
from a UUID column, then that really needs to happen; if there's no
reference, then we're probably starting from a row in the wrong table.

This fixes an assertion failure in command "ovs-vsctl list netflow br0",
if bridge br0 without any netflows.
$ovs-vsctl list netflow br0
ovs-vsctl: lib/ovsdb-idl.c:2407: assertion column_idx < class->n_columns failed
in ovsdb_idl_read()
Aborted

Fixes: 3f5b5f7b4115 ("db-ctl-base: Always support all tables in schema.")
Signed-off-by: Zhipeng Lu <lu.zhipeng@zte.com.cn>
Signed-off-by: Ben Pfaff <blp@ovn.org>