fix backwards subtraction

libs/stfu/stfu.c

@@ -718,8 +718,8 @@ stfu_frame_t *stfu_n_read_a_frame(stfu_instance_t *i)
         uint32_t y;
         stfu_frame_t *frame = NULL;
 
-        int32_t delay = i->last_rd_ts - i->cur_ts;
-        uint32_t need  = abs(i->last_rd_ts - i->cur_ts) / i->samples_per_packet;
+        int32_t delay = i->cur_ts - i->last_rd_ts;
+        uint32_t need  = abs(delay) / i->samples_per_packet;
 
         
         i->period_missing_count++;