[NETFILTER]: Fix crash in ip_nat_pptp

When an inbound PPTP_IN_CALL_REQUEST packet is received the
PPTP NAT helper uses a NULL pointer in pointer arithmentic to
calculate the offset in the packet which needs to be mangled
and corrupts random memory or crashes.

Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/netfilter/ip_nat_helper_pptp.c

···
 		break;
 	case PPTP_IN_CALL_REQUEST:
 		/* only need to nat in case PAC is behind NAT box */
-		break;
+		return NF_ACCEPT;
 	case PPTP_WAN_ERROR_NOTIFY:
 		pcid = &pptpReq->wanerr.peersCallID;
 		break;