mozilla/source-map — pull request #131 by chmontgomery (2014-08-29 17:45:28+00:00)
Title: 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