[NETFILTER]: nf_nat_sip: don't drop short packets

Don't drop packets shorter than "SIP/2.0", just ignore them. Keep-alives
can validly be shorter for example.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/netfilter/nf_nat_sip.c

@@ -104,7 +104,7 @@ static unsigned int ip_nat_sip(struct sk_buff **pskb,
 	dataoff = ip_hdrlen(*pskb) + sizeof(struct udphdr);
 	datalen = (*pskb)->len - dataoff;
 	if (datalen < sizeof("SIP/2.0") - 1)
-		return NF_DROP;
+		return NF_ACCEPT;
 
 	addr_map_init(ct, &map);