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