captureplan: sip_check() fix for harmless error logging

[dougbtv]

So I noticed when using sip_check() in capture plan to limit what messaging to capture, that there's some harmless error messages in the logs, I think there's a missing else-if in protocol_sip.c

If you're doing something like sip_check("method","REGISTER") in a capture plan, you'll get a harmless error stating:

[ERR] protocol_sip.c:195 unknown variable [method]

Need an else-if here:

captagent/src/modules/protocol/sip/protocol_sip.c

Line 170 in 84869b7

}

Otherwise you get this error message:

captagent/src/modules/protocol/sip/protocol_sip.c

Line 195 in 84869b7

LERR("unknown variable [%s]\n", param1);

I'll submit a PR as soon as I get a chance. It's not breaking, but, it appears as an error when you're kind of not expecting it.

[dougbtv]

Closed up in #69