pgsql: Fix compiler warning in back branches (9.6, 10).

Fix compiler warning in back branches (9.6, 10).

Back-patch a tiny bit of commit fbb2e9a0 into 9.6 and 10, to silence an
uninitialized variable warning from GCC 10.2.  Seen on buildfarm member
handfish, and my own development workflow where I like to use -Werror.

Discussion: https://postgr.es/m/CA%2BhUKGJRcwvK86Uf5t-FrTekZjqHtpv3u%3D3MuBg8Zw8R933Mqg%40mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/618d139f82d4222df796e989de3c4c4de0359d7b

Modified Files
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src/backend/utils/misc/guc.c | 2 ++
1 file changed, 2 insertions(+)