[NETFILTER]: Don't assign new helper after NAT when there are already expectations...

[NETFILTER]: Don't assign new helper after NAT when there are already expectations present.

Tracked down by Raivis Bucis <raivis@mt.lv>

This patch fixes an oops while listing /proc/net/ip_conntrack.

When a helper sets up expectations based on the first packet (tftp),
NAT can still change the packet and cause conntrack to look for a new helper
based on the new tuple. When no helper is found, expectant->helper will be
NULL, which leads to an oops in print_expect().

Only assign a new helper in ip_conntrack_alter_reply() if there are no
expectations.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@redhat.com>