pgsql: 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(dot)vf(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAEudQAonQh7be=wOR-CJFW=bgMBz5wW_bv4t0OFxbgn-794JCQ@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9a2c07cbde9796049e4b37e3dde0b6f5253cb97b

Modified Files
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src/backend/utils/adt/multixactfuncs.c | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)