prioq: fix index range check

src/basic/prioq.c

@@ -217,7 +217,7 @@ _pure_ static struct prioq_item* find_item(Prioq *q, void *data, unsigned *idx)
 
         if (idx) {
                 if (*idx == PRIOQ_IDX_NULL ||
-                    *idx > q->n_items)
+                    *idx >= q->n_items)
                         return NULL;
 
                 i = q->items + *idx;