Lookup object UID for Sequence generator

The NATable object representing a Sequence generator sometimes has
an object UID of 0 at the time privileges are checked, resulting
in failure to locate the object and denial of access. This change
looks up the UID if the corresponding NATable member variable is 0.

Change-Id: I889d2d823b8c0faca5318572250b45e07134b6cf
Merge "Lookup object UID for Sequence generator"