fix bug on `SectionParser.h`

test crashes when running [`test-Indentation.cc`](https://github.com/apiaryio/snowcrash/blob/shared/refactor-integration/test/test-Indentation.cc#L77) test case `No Indentation` on windows due to the calling the property of an `end` iterator.

cc. @zdne 
@alikh31 please make it in one condition, also **follow** the [Coding Standards](https://github.com/apiaryio/snowcrash/wiki/Coding-Standards#style-in-snow-crash)!