fix long line parsing

Was: comparing unsigned for negative values; substracting 0 from 'size'
variable; crash on long line in parsed file.

Becomes: reduce unsigned variable 'size' properly, handle long lines.

BTW, I've set up a regular checking of Instead (and a few other projects) with CppCheck and generating reports in html. This issue was found by analyzer. You can found reports for Instead here: http://begorov.me/reports/instead/