[NETFILTER]: nf_conntrack_netbios_ns: fix uninitialized member in expectation

->helper is uninitialized in the expectation registered by the netbios_ns
helper and it later copied to the expected connection, which causes invalid
memory dereferences when trying to call the helper.

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