fixing yet another bug (cf bug#334): ModularBigInteger.pow give wrong results for negative exponents

Author: manfredscheucher

bignum/src/commonMain/kotlin/com/ionspin/kotlin/bignum/modular/ModularBigInteger.kt

@@ -229,6 +229,8 @@ class ModularBigInteger private constructor(
         var e = exponent
         return if (this.modulus == BigInteger.ONE) {
             creator.ZERO
+        } else if (e < 0) {
+            return inverse().pow(-e)
         } else {
             var residue = BigInteger.ONE
             var base = this.residue