Fix scope issues with service related variables

[nicolasbadia]

I had this error with the current master branch:

events.js:291
      throw er; // Unhandled 'error' event
      ^

ReferenceError: service is not defined
    at /opt/gestibox/node_modules/node-nmap/index.js:91:11
    at Array.map (<anonymous>)
    at /opt/gestibox/node_modules/node-nmap/index.js:70:37
    at Array.map (<anonymous>)
    at convertRawJsonToScanResults (/opt/gestibox/node_modules/node-nmap/index.js:34:27)
    at /opt/gestibox/node_modules/node-nmap/index.js:238:19
    at Parser.<anonymous> (/opt/gestibox/node_modules/xml2js/lib/parser.js:304:18)
    at Parser.emit (events.js:314:20)
    at SAXParser.onclosetag (/opt/gestibox/node_modules/xml2js/lib/parser.js:262:26)
    at emit (/opt/gestibox/node_modules/sax/lib/sax.js:624:35)
Emitted 'error' event on Parser instance at:
    at Parser.exports.Parser.Parser.parseString (/opt/gestibox/node_modules/xml2js/lib/parser.js:327:16)
    at Parser.parseString (/opt/gestibox/node_modules/xml2js/lib/parser.js:5:59)
    at Object.exports.parseString (/opt/gestibox/node_modules/xml2js/lib/parser.js:369:19)
    at NmapScan.rawDataHandler (/opt/gestibox/node_modules/node-nmap/index.js:232:12)
    at ChildProcess.<anonymous> (/opt/gestibox/node_modules/node-nmap/index.js:209:14)
    at ChildProcess.emit (events.js:314:20)
    at maybeClose (internal/child_process.js:1022:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:287:5)