clear the pointer after freeing the mbuf. Without that, we

risk a double free if the subsequent mbuf allocation fails.
This bug is not netmap-related and was introduced in  rev. 228387

sys/dev/e1000/if_em.c

@@ -4044,6 +4044,7 @@ em_setup_receive_ring(struct rx_ring *rxr)
 			    BUS_DMASYNC_POSTREAD);
 			bus_dmamap_unload(rxr->rxtag, rxbuf->map);
 			m_freem(rxbuf->m_head);
+			rxbuf->m_head = NULL; /* mark as freed */
 		}
 	}