[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 <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_nat_helper_pptp.c

@@ -315,7 +315,7 @@ pptp_inbound_pkt(struct sk_buff **pskb,
 		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;