Fix a potential bug of ParallelLoopBodyWrapper::operator(Range)
On a 32-bit compiler the calculation may result in data (size_t) overflow when running some paralleled algorithms (which can safely run on a 64-bit compiler).
This bug is found when running OpenCV's Retina tutorial on 32bit VS2010.
Merge pull request #1332 from pengx17:2.4_fix_parallel