Fix incorrect early exit from contain_mutable_functions_walker.

Given a JsonExpr.path_spec that is a null constant,
contain_mutable_functions_walker immediately returned false, failing
to check for other sources of mutability elsewhere in the JsonExpr.
It must only skip the jspIsMutable() test in this case.

While here, remove a couple of redundant castNode calls (I'm not
a fan of using that directly after an explicit IsA check).

Bug: #19840
Reported-by: chunling qin <303677365@qq.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19840-7d5243be84d7ecd3@postgresql.org
Backpatch-through: 17

Author: tglsfdc

src/backend/optimizer/util/clauses.c

@@ -445,26 +445,26 @@ contain_mutable_functions_walker(Node *node, void *context)
 				return true;
 		}
 
-		/* Check all subnodes */
+		/* Fall through to check subnodes */
 	}
 
 	if (IsA(node, JsonExpr))
 	{
-		JsonExpr   *jexpr = castNode(JsonExpr, node);
+		JsonExpr   *jexpr = (JsonExpr *) node;
 		Const	   *cnst;
 
+		/* If non-constant path_spec, must treat as mutable */
 		if (!IsA(jexpr->path_spec, Const))
 			return true;
 
-		cnst = castNode(Const, jexpr->path_spec);
-
+		cnst = (Const *) jexpr->path_spec;
 		Assert(cnst->consttype == JSONPATHOID);
-		if (cnst->constisnull)
-			return false;
-
-		if (jspIsMutable(DatumGetJsonPathP(cnst->constvalue),
+		if (!cnst->constisnull &&
+			jspIsMutable(DatumGetJsonPathP(cnst->constvalue),
 						 jexpr->passing_names, jexpr->passing_values))
 			return true;
+
+		/* Fall through to check subnodes */
 	}
 
 	if (IsA(node, SQLValueFunction))