netfilter: nf_ct_sip: fix helper name

Commit 3a8fc53a (netfilter: nf_ct_helper: allocate 16 bytes for the helper
and policy names) introduced a bug in the SIP helper, the helper name is
sprinted to the sip_names array instead of instead of into the helper
structure. This breaks the helper match and the /proc/net/nf_conntrack_expect
output.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>