FIX: IO#sysread occurs SEGV

```
% ./mruby -e 'File.open("/dev/zero").sysread(100000)'
zsh: segmentation fault (core dumped)  ./mruby -e 'File.open("/dev/zero").sysread(100000)'
```

If the 2nd argument of `mrb_str_new()` is not null, `mrb_str_new()` tries to copy from the 2nd argument.