Fix a bug when file contains a line that only contains whitespace

Editorconfig visual studio extension will will fail to process a file that contains a line that only contains white spaces.
![image](https://cloud.githubusercontent.com/assets/4053944/4184569/a1190474-374b-11e4-9abb-b16c215be4d4.png)

When processing a line only contains whitespaces, pos will become -1 when reached beginning of the line. Then content[-1] will cause a out of index exception.