netfilter: nf_nat_sip: c= is optional for session

According to RFC2327, the connection information is optional
in the session description since it can be specified in the
media description instead.

My provider does exactly that and does not provide any connection
information in the session description.  As a result the new
kernel drops all invite responses.

This patch makes it optional as documented.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>