py: Fix bug in optimised for .. range.

Don't store final, failing value to the loop variable.  This fix also
makes for .. range a bit more efficient, as it uses less store/load
pairs for the loop variable.

tests/basics/for2.py

@@ -0,0 +1,8 @@
+i = 'init'
+for i in range(0):
+    pass
+print(i) # should not have been modified
+
+for i in range(10):
+    pass
+print(i) # should be last successful value of loop