pgsql: Fix crash when using COLLATE in partition bound expressions

Fix crash when using COLLATE in partition bound expressions

Attempting to use a COLLATE clause with a type that it not collatable in
a partition bound expression could crash the server.  This commit fixes
the code by adding more checks similar to what is done when computing
index or partition attributes by making sure that there is a collation
iff the type is collatable.

Backpatch down to 12, as 7c079d7 introduced this problem.

Reported-by: Alexander Lakhin
Author: Dmitry Dolgov
Discussion: https://postgr.es/m/16325-809194cf742313ab@postgresql.org
Backpatch-through: 12

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0a9ae1800f67339a8acb45029369e60b98e72fd6

Modified Files
--------------
src/backend/parser/parse_utilcmd.c         | 24 ++++++++++++++++++++++++
src/test/regress/expected/create_table.out |  6 ++++++
src/test/regress/sql/create_table.sql      |  2 ++
3 files changed, 32 insertions(+)