Initialize new jsonb iterator to zero

Use palloc0() instead of palloc() to create a new JsonbIterator.
Otherwise, the isScalar field is sometimes not initialized.  There is
probably no impact in practice, but it's cleaner this way and it avoids
future problems.

src/backend/utils/adt/jsonb_util.c

@@ -901,7 +901,7 @@ iteratorFromContainer(JsonbContainer *container, JsonbIterator *parent)
 {
 	JsonbIterator *it;
 
-	it = palloc(sizeof(JsonbIterator));
+	it = palloc0(sizeof(JsonbIterator));
 	it->container = container;
 	it->parent = parent;
 	it->nElems = JsonContainerSize(container);