[NETFILTER]: SIP conntrack: fix out of bounds memory access

When checking for an @-sign in skp_epaddr_len, make sure not to
run over the packet boundaries.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_conntrack_sip.c

@@ -292,7 +292,7 @@ static int skp_epaddr_len(const char *dptr, const char *limit, int *shift)
 		dptr++;
 	}
 
-	if (*dptr == '@') {
+	if (dptr <= limit && *dptr == '@') {
 		dptr++;
 		(*shift)++;
 	} else

net/netfilter/nf_conntrack_sip.c

@@ -312,7 +312,7 @@ static int skp_epaddr_len(struct nf_conn *ct, const char *dptr,
 		dptr++;
 	}
 
-	if (*dptr == '@') {
+	if (dptr <= limit && *dptr == '@') {
 		dptr++;
 		(*shift)++;
 	} else