res_pjsip_endpoint_identifier_ip: Ensure error defaults to 0.

When configuring a match using a netmask the error variable was
not defaulting to 0. For some people this would cause the code
to think an error occurred when adding the match when in reality
it added perfectly fine.

ASTERISK-26693

Change-Id: I850c250813742bddde65c84e739093c9e01dfe56