pgsql: Fix bug in DROP OWNED BY.

Fix bug in DROP OWNED BY.

Commit 6566133c5f52771198aca07ed18f84519fac1be7 broke the case where
the role passed to DROP OWNED BY owns a database.

Report by Rushabh Lathia, who also provided a patch, but this patch
takes a slightly different approach to fixing the problem.

Discussion: http://postgr.es/m/CAGPqQf2vO+nbo=3yAdZ8v26Rbug7bY4YjPaPLZx=L1NZ9-CC3w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7188b9b0fdbb4dd99915626f171d820a56b98514

Modified Files
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src/backend/catalog/pg_shdepend.c | 29 +++++++++++++++++++----------
src/bin/scripts/t/020_createdb.pl | 18 ++++++++++++++++++
2 files changed, 37 insertions(+), 10 deletions(-)