[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: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netfilter/nf_conntrack_sip.c b/net/netfilter/nf_conntrack_sip.c
index c93fb37..9dec115 100644
--- a/net/netfilter/nf_conntrack_sip.c
+++ b/net/netfilter/nf_conntrack_sip.c
@@ -312,7 +312,7 @@
 		dptr++;
 	}
 
-	if (*dptr == '@') {
+	if (dptr <= limit && *dptr == '@') {
 		dptr++;
 		(*shift)++;
 	} else