pgsql: Ensure pg_filenode_relation(0, 0) returns NULL.

Ensure pg_filenode_relation(0, 0) returns NULL.

Previously, a zero value for the relfilenode resulted in
a confusing error message about "unexpected duplicate".
This function returns NULL for other invalid relfilenode
values, so zero should be treated likewise.

It's been like this all along, so back-patch to all supported
branches.

Justin Pryzby

Discussion: https://postgr.es/m/20210612023324.GT16435@telsasoft.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c1ffbbcbca93b3544c0b57b7ab063497794cb8ff

Modified Files
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src/backend/utils/adt/dbsize.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)