Rework locking code in GetMultiXactIdMembers
After commit
53c2a97a9266, the code flow around the "retry" goto label
in GetMultiXactIdMembers was confused about what was possible: we never
return there with a held lock, so there's no point in testing for one.
This realization lets us simplify the code a bit. While at it, make the
scope of a couple of local variables in the same function a bit tighter.
Per Coverity.