Fix exponentiation operation to avoid overflow.

The exponentation implementation unnecessarily multiplied the
result with itself at the end if the exponent was an even number.
This led to overflow if result*result > high(int).

lib/pure/math.nim

@@ -427,10 +427,12 @@ proc `^`*[T](x, y: T): T =
   var (x, y) = (x, y)
   result = 1
 
-  while y != 0:
+  while true:
     if (y and 1) != 0:
       result *= x
     y = y shr 1
+    if y == 0:
+      break
     x *= x
 
 proc gcd*[T](x, y: T): T =