pgsql: Fix assertion failure in CREATE DATABASE

Fix assertion failure in CREATE DATABASE

An assertion would fail when creating a database with libc locale
provider from a template database with icu locale provider.

Reported-by: Marina Polyakova <m.polyakova@postgrespro.ru>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: /messages/by-id/f385ba25e7f8be427b8c582e5cca7d79@postgrespro.ru

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a2caf182a2edcee7b1c00036b7dba6762941473c

Modified Files
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src/backend/commands/dbcommands.c | 4 ++--
src/bin/scripts/t/020_createdb.pl | 9 +++++++++
2 files changed, 11 insertions(+), 2 deletions(-)