netfilter: nf_conntrack_sip: fix ct_sip_parse_request() REGISTER request parsing

When requests are parsed, the "sip:" part of the SIP URI should be skipped.

Usually this doesn't matter because address parsing skips forward until after
the username part, but in case REGISTER requests it doesn't contain a username
and the address can not be parsed.

Signed-off-by: Patrick McHardy <kaber@trash.net>