pgsql: Fix planner failure with extended statistics on partitioned tabl

Fix planner failure with extended statistics on partitioned tables.

Some cases would result in "cache lookup failed for statistics object",
due to trying to fetch inherited statistics when only non-inherited
ones are available or vice versa.

Richard Guo and Justin Pryzby

Discussion: https://postgr.es/m/20221030170520.GM16921@telsasoft.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1f1865e9083625239769c26f68b9c2861b8d4b1c

Modified Files
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src/backend/utils/adt/selfuncs.c        | 11 +++++++++--
src/test/regress/expected/stats_ext.out |  8 +++++++-
src/test/regress/sql/stats_ext.sql      |  3 ++-
3 files changed, 18 insertions(+), 4 deletions(-)