sca: fix core when HDR_CSEQ not parsed

[linuxmaniac]

Program terminated with signal SIGSEGV, Segmentation fault.

0 0x00007f1820227bae in sca_call_info_update (msg=0x7f1828635930, p1=0x0, p2=0x0, p3=0x7f1828635930 "\001") at sca_call_info.c:1845

1845 LM_DBG("BUG: sca module does not support Call-Info headers "
(gdb) p msg->cseq
$1 = (struct hdr_field *) 0x0
(gdb) p msg->first_line.u.request->method
$2 = {
s = 0xa33540 "SUBSCRIBE sip:30@127.0.0.1:5060 SIP/2.0\r\nVia: SIP/2.0/UDP 172.17.0.2:5061;branch=z9hG4bK-2884-1-0\r\nFrom: sipp sip:sipp@172.17.0.2:5061;tag=2884SIPpTag001\r\nTo: sut sip:30@127.0.0.1:5060\r\nCall-ID: 1"..., len = 9}