Fix the only static analysis issue left

The logic seems to be correct in code, but static analysis keeps reporting this issue:

http://librecad.github.io/static-analyzer-reports/

I suppose there's one elegant way to silence the warning and IMPROVE the code.

Could it be because common.pri is included, which sets C++11?
https://github.com/LibreCAD/LibreCAD/blob/master/plugins/importshp/importshp.pro

...since it is a .c file?