pgsql: Fix search_path cache initialization.

Fix search_path cache initialization.

The cache needs to be available very early, so don't rely on
InitializeSearchPath() to initialize the it.

Reported-by: Murat Efendioğlu
Discussion: https://postgr.es/m/CACbCzujQ4zS8MM1bx-==+tr+D3Hk5G1cjN4XkUQ+Q=cEpwhzqg@mail.gmail.com
Backpatch-through: 17

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a9ed7d944965865e5c6b44e58602f621cd185856

Modified Files
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src/backend/catalog/namespace.c | 28 +++++++++++++++++++---------
1 file changed, 19 insertions(+), 9 deletions(-)