pgsql: Silence compiler warnings from some older compilers.

Silence compiler warnings from some older compilers.

Since a117cebd6, some older gcc versions issue "variable may be used
uninitialized in this function" complaints for brin_summarize_range.
Silence that using the same coding pattern as in bt_index_check_internal;
arguably, a117cebd6 had too narrow a view of which compilers might give
trouble.

Nathan Bossart and Tom Lane. Back-patch as the previous commit was.

Discussion: https://postgr.es/m/20220601163537.GA2331988@nathanxps13

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/dd1c8dd1015f4548d6c29de56e54eda317b39319

Modified Files
--------------
contrib/amcheck/verify_nbtree.c | 2 +-
src/backend/access/brin/brin.c | 6 ++++++
2 files changed, 7 insertions(+), 1 deletion(-)