fix TypeError when sourceContents is already deleted

I'm seeing the following error when using `gulp-sourcemaps`, `gulp-less` and `gulp-concat` together:

```
/Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/gulp-concat/node_modules/concat-with-sourcemaps/node_modules/source-map/lib/source-map/source-map-generator.js:143
        delete this._sourcesContents[util.toSetString(source)];
                                          ^
TypeError: Cannot convert null to object
    at SourceMapGenerator_setSourceContent [as setSourceContent] (/Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/gulp-concat/node_modules/concat-with-sourcemaps/node_modules/source-map/lib/source-map/source-map-generator.js:143:43)
    at /Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/gulp-concat/node_modules/concat-with-sourcemaps/index.js:63:28
    at Array.forEach (native)
    at Concat.add (/Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/gulp-concat/node_modules/concat-with-sourcemaps/index.js:62:35)
    at Stream.bufferContents (/Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/gulp-concat/index.js:25:12)
    at Stream.stream.write (/Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/gulp-concat/node_modules/through/index.js:26:11)
    at write (/Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/readable-stream/lib/_stream_readable.js:605:24)
    at flow (/Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/readable-stream/lib/_stream_readable.js:614:7)
    at Transform.pipeOnReadable (/Users/montgomeryc/Projects/gulp-bundle-assets/node_modules/readable-stream/lib/_stream_readable.js:646:5)
    at Transform.emit (events.js:92:17)
```

Unsure why this is exactly happening but my change seems unobtrusive to the logic flow and results in my compiled less file and source map being created properly.

/cc @contra