af-packet: leave reading loop at each turn

The idea of this patch is to be sure to leave the ring reading loop
enough to be able to sync counters. This should fix #706.

src/source-af-packet.c

@@ -838,6 +838,8 @@ int AFPReadFromRing(AFPThreadVars *ptv)
 next_frame:
         if (++ptv->frame_offset >= ptv->req.tp_frame_nr) {
             ptv->frame_offset = 0;
+            /* Get out of loop to be sure we will reach maintenance tasks */
+            SCReturnInt(AFP_READ_OK);
         }
     }