Bounds-check after arithmetic is complete

double_to_size can return std::numeric_limits<size_t>max(), so we cannot add 1 to the return value of double_to_size. That addition should be done while still working with a double, as can be seen being done on line 850 of this file.
This was uncovered by Coverity, and addresses Coverity issues CID13443 and CID12664
Merge pull request #4 from tempoz/patch-2

Bounds-check after arithmetic is complete