pgsql: postgres_fdw: Fix "may be used uninitialized" warning in foreign

postgres_fdw: Fix "may be used uninitialized" warning in foreign_join_ok()

Commit 0ee83dd4a99 detected the mixed foreign x function-RTE cases in one
if/else chain that only set a pair of bool flags, then acted on those flags in
a second chain. fpinfo_o and fpinfo_i are each assigned in one chain and read
in the other, so proving them initialized requires correlating the flags with
the assignments. gcc 13 with -Og does not manage that and reports fpinfo_i as
possibly uninitialized; clang's -Wconditional-uninitialized likewise reports
both variables.

The flags served no purpose beyond deferring the work, so merge the two chains
and read both fdw_private pointers up front, unconditionally. A function rel
never has an fdw_private of its own, so the pointer is simply NULL until the
stub replaces it, and each branch now assigns and uses the two variables in one
place.

Reported-by: Karina Litskevich <litskevichkarina(at)gmail(dot)com>
Reported-by: Marina Polyakova <m(dot)polyakova(at)postgrespro(dot)ru>
Reviewed-by: Marina Polyakova <m(dot)polyakova(at)postgrespro(dot)ru>
Discussion: https://postgr.es/m/CACiT8iamoL-%3D792e6JK2uCVvwDhfCg7DXHhvqN0P33jhGg2-8A%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0fb258d1f2ca65b7299ba7c47045634df05efbff

Modified Files
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contrib/postgres_fdw/postgres_fdw.c | 53 +++++++++++++------------------------
1 file changed, 19 insertions(+), 34 deletions(-)