Handle really big steps in extended slices.

Fixes a test failure on 64 bit platforms (I hope).
diff --git a/Python/ceval.c b/Python/ceval.c
index afc480e..09b88a6 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -3507,7 +3507,7 @@
 		if (x > INT_MAX)
 			x = INT_MAX;
 		else if (x < -INT_MAX)
-			x = 0;
+			x = -INT_MAX;
 		*pi = x;
 	}
 	return 1;