8215759: java/math/BigInteger/ModPow.java can throw an ArithmeticException

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8215759
http://cr.openjdk.java.net/~bpb/8215759/webrev.00/

Although highly unlikely, the modulus parameter of modPow() can be zero which would provoke an ArithmeticException.
Unsure whether I need this bug ID in the @bug list.

Thanks,

Brian