new java.lang.Bignum(Long.MIN_VALUE) creates Bignum object
with value -1237940030062008238044348416 instead of -9223372036854775808

Minimized test demonstrating the bug:

 class java_lang_Bignum_constructor_2 {
  public static void main(String args[]) {
    System.out.println(new java.lang.Bignum(Long.MIN_VALUE));
    // will print "-1237940030062008238044348416"
    // instead of "-9223372036854775808"
  }
}