Avoid useless calls in pg_get_multixact_stats()

Avoid useless calls in pg_get_multixact_stats()

MultiXactOffsetStorageSize() and GetMultiXactInfo() are called to gather
the information reported by the function, but were wasteful for the case
where a role does not have the privileges of pg_read_all_stats, where we
return a set of NULLs.  These calls are moved to the code path where
their results are used.

Author: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://postgr.es/m/CAEudQAonQh7be=wOR-CJFW=bgMBz5wW_bv4t0OFxbgn-794JCQ@mail.gmail.com
Backpatch-through: 19