Fix MSVC warnings for numeric.c

This fix suppress warning C4756 below in MSVC(<VC++2013)

```
e:\work\mruby\mruby\src\numeric.c(307) : warning C4756: 定数演算でオーバーフローを起こしました。
e:\work\mruby\mruby\src\numeric.c(816) : warning C4756: 定数演算でオーバーフローを起こしました。
```

Other method to suppress this warning is to disable this warning only in numeric.c( MSVC has such ability : `#pragma warning (disable:C4765)`).
But It would introduce  #ifdef _MSC_VER,,, lines in numeric.c. I think it is better to avoid such annoying compiler specific #ifdef in *.c.