plugin: fetchinvoice: assign the quantity to the invrequest field

assign the quantity field receive as RPC parameter inside the
invrequest field.

Changelog-Fixed: fetchinvoice: assign the quantity to the invrequest field

Reported-by: @aaronbarnardsound
Link: #6089
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

plugins/fetchinvoice.c

@@ -1029,6 +1029,7 @@ static struct command_result *json_fetchinvoice(struct command *cmd,
 	invreq = invoice_request_for_offer(sent, sent->offer);
 	invreq->invreq_recurrence_counter = tal_steal(invreq, recurrence_counter);
 	invreq->invreq_recurrence_start = tal_steal(invreq, recurrence_start);
+	invreq->invreq_quantity = tal_steal(invreq, quantity);
 
 	/* BOLT-offers-recurrence #12:
 	 * - if `offer_amount` is not present: