pgsql: Fix get_object_namespace() not to think extensions are "in" a sc

Fix get_object_namespace() not to think extensions are "in" a schema.

extnamespace means something altogether different in this context.
Mostly by accident, this coding error (introduced in my commit
82a4a777d94bec965ab2f1d04b6e6a3f0447b377) broke the buildfarm instead
of just silently doing the wrong thing.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8f3362d4b7430079bb53a954e372955eb285cf52

Modified Files
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src/backend/catalog/objectaddress.c |    2 +-
1 files changed, 1 insertions(+), 1 deletions(-)