Avoid dangling-pointer problem with partitionwise joins under GEQO.

build_child_join_sjinfo creates a derived SpecialJoinInfo in
the short-lived GEQO context, but afterwards the semi_rhs_exprs
from that may be used in a UniquePath for a child base relation.
This breaks the expectation that all base-relation-level structures
are in the planning-lifespan context, leading to use of a dangling
pointer with probable ensuing crash later on in create_unique_plan.
To fix, copy the expression trees when making a UniquePath.

Per bug #18360 from Alexander Lakhin.  This has been broken since
partitionwise joins were added, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/18360-a23caf3157f34e62@postgresql.org

src/backend/optimizer/util/pathnode.c

@@ -1707,8 +1707,13 @@ create_unique_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->path.pathkeys = NIL;
 
 	pathnode->subpath = subpath;
-	pathnode->in_operators = sjinfo->semi_operators;
-	pathnode->uniq_exprs = sjinfo->semi_rhs_exprs;
+
+	/*
+	 * Under GEQO, the sjinfo might be short-lived, so we'd better make copies
+	 * of data structures we extract from it.
+	 */
+	pathnode->in_operators = copyObject(sjinfo->semi_operators);
+	pathnode->uniq_exprs = copyObject(sjinfo->semi_rhs_exprs);
 
 	/*
 	 * If the input is a relation and it has a unique index that proves the