delegate boundary cases to existing IntInf.pow;

--- a/src/Pure/General/integer.ML	Fri Nov 03 17:27:00 2017 +0100
+++ b/src/Pure/General/integer.ML	Fri Nov 03 19:16:41 2017 +0100
@@ -51,7 +51,7 @@
           in if r = 0 then l' else l' * l end;
   in
     if k < 0
-    then error "pow: negative exponent"
+    then IntInf.pow (l, k)
     else pw k l
   end;