new java.lang.Bignum(Integer.MIN_VALUE) creates Bignum object
with value -1152921502459363328 instead of -21474483648

Minimized test demonstrating the bug:

class java_lang_Bignum_constructor_1 {
  public static void main(String args[]) {
    System.out.println(new java.lang.Bignum(Integer.MIN_VALUE));
    // will print "-1152921502459363328" instead of "-21474483648"
  }
}