Check that the error object (e) is not null

This fixes the followint error 

```
#51 Too many errors. (20% scanned).
      // Line 33, Pos 4

/usr/lib/node_modules/jslint/lib/reporter.js:46
                    line = ' // Line ' + e.line + ', Pos ' + e.character;
                                          ^
TypeError: Cannot read property 'line' of null
    at /usr/lib/node_modules/jslint/lib/reporter.js:46:43
    at Array.forEach (native)
    at Object.exports.report (/usr/lib/node_modules/jslint/lib/reporter.js:41:29)
    at Object.report (/usr/lib/node_modules/jslint/lib/reporter.js:21:39)
    at ReportStream_constructor.ReportStream_transform [as _transform] (/usr/lib/node_modules/jslint/lib/reportstream.js:48:23)
    at ReportStream_constructor.Transform._read (/usr/lib/node_modules/jslint/node_modules/readable-stream/lib/_stream_transform.js:184:10)
    at ReportStream_constructor.Transform._write (/usr/lib/node_modules/jslint/node_modules/readable-stream/lib/_stream_transform.js:172:12)
    at doWrite (/usr/lib/node_modules/jslint/node_modules/readable-stream/lib/_stream_writable.js:238:10)
    at writeOrBuffer (/usr/lib/node_modules/jslint/node_modules/readable-stream/lib/_stream_writable.js:228:5)
    at ReportStream_constructor.Writable.write (/usr/lib/node_modules/jslint/node_modules/readable-stream/lib/_stream_writable.js:195:11)
```
This problem exists versions  0.4.0, 0.4.1 and 0.5.0 and most probably is related to #73