Adjust ci->nregs to prevent heap corruption

```bash 
bin/mruby -e "fib = Hash.new{ |h,k| h[k] = k < 2 ? k : h[k-1] + h[k-2] }; \
puts fib[18]" 
*** glibc detected *** bin/mruby: realloc(): invalid next size: 0x081a3b28 ***
```

p->body.irep->nregs is 0 (not sure why) so stack is not extended. But argc is 2, so stack should have been extended by 4. 

If this fix is OK, there is another issue of a stack overflow caused by running this code with n > 6000.