res_pjsip_pubsub: Increment persistence data ref when recreating.

Each subscription needs to have a reference to the persisted data
for it, as well as the main JSON contained within the tree. When
recreating a subscription this did not occur and they both shared
the same reference.

ASTERISK-28714

Change-Id: I706abd49ea182ea367a4ac3feca2706460ae9f4a
(cherry picked from commit 35c9332edf)

res/res_pjsip_pubsub.c

@@ -1264,7 +1264,7 @@ static struct ast_sip_subscription *allocate_subscription(const struct ast_sip_s
 	 */
 
 	if (tree->persistence && tree->persistence->generator_data) {
-		sub->persistence_data = ast_json_object_get(tree->persistence->generator_data, resource);
+		sub->persistence_data = ast_json_ref(ast_json_object_get(tree->persistence->generator_data, resource));
 	}
 
 	sub->handler = handler;