TestReader: fix -Wformat warnings
TestReader: fix -Wformat warnings

The recent loadpng commit introduced some warnings about printf type mismaches. I assume the changes were supposed to fix the same warnings on another system (where size_t is int instead of long?). Use explicit casts to int to fix it for both environments.

huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp,huycn/zxing-cpp,nu-book/zxing-cpp