cast literal to csize to fix error

On Ubuntu 13.04 x64 with gcc/g++ 4.7.3
```
/data/dyu/dev/gh/flatbuffers/tests/test.cpp:140:3:   required from here
/data/dyu/dev/gh/flatbuffers/tests/test.cpp:40:3: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
```