SourceMap: Don't entertain garbage data (#586)

It turned out to be `std::bad_alloc` exception. When source-map column is [*not* updated](https://github.com/sass/libsass/blob/7a381b892941d5d8d66b8a3740aab880c0a2e79f/output_nested.cpp#L265-L266) node-x86 on Windows x64 throws.

Turned out, the `generated_position.line` was getting garbage value: `std::numeric_limits<size_t>::max()` (4294967295) and its variants: sometimes 4294967293 or 4294967292.

This fixes gh-586. :bulb: 

/cc @mgreter, @xzyfer 
IMO this is not correct and should look like that instead (??):
`result += std::string(generated_line - previous_generated_line, ';');`