When exponent is negative, this Bignum is 0, method should throw
ArithmeticException but throws ArrayIndexOutOfBoundsException
==== Here is the minimzied test demonstrating the bug ====
import java.lang.Bignum;
class java_lang_Bignum_modExp {
  public static void main(String args[]) {
     Bignum b1=new Bignum(0);
     Bignum b2=new Bignum(-5);
     Bignum b3=new Bignum(3);
     System.out.println(b1.modExp(b2,b3));
  }
}
==== Here is the output of the test ====
java.lang.ArrayIndexOutOfBoundsException: -1
at java.lang.Bignum.mul0(Bignum.java)
at java.lang.Bignum.modExp(Bignum.java)
at java_lang_Bignum_modExp.main(java_lang_Bignum_modExp.java:7)