net/gnrc/tcp: fix uninitialized memory access

From the gnrc_pktbuf_mark documentation:

	    It's not guaranteed that `result->data` points to the
	    same address as the original `pkt->data.

Thus it should be necessary to update the `tcp` pointer.

sys/net/gnrc/transport_layer/tcp/gnrc_tcp_eventloop.c

@@ -162,6 +162,7 @@ static int _receive(gnrc_pktsnip_t *pkt)
             return -ENOMSG;
         }
         pkt->type = GNRC_NETTYPE_UNDEF;
+        hdr = (tcp_hdr_t *)tcp->data;
     }
 
     /* Validate checksum */