[NETFILTER]: SIP conntrack: fix skipping over user info in SIP headers

When trying to skip over the username in the Contact header, stop at the
end of the line if no @ is found to avoid mangling following headers.
We don't need to worry about continuation lines because we search inside
a SIP URI.

Fixes Netfilter Bugzilla #532.

Signed-off-by: Lars Immisch <lars@ibp.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>