Patch 1012740: cStringIO's truncate doesn't

truncate() left the stream position unchanged, which meant the
"truncated" data didn't go away:

>>> io.write('abc')
>>> io.truncate(0)
>>> io.write('xyz')
>>> io.getvalue()
'abcxyz'

Patch by Dima Dorfman.

Modules/cStringIO.c

@@ -289,6 +289,7 @@ IO_truncate(IOobject *self, PyObject *args) {
         if (pos < 0) pos = self->pos;
 
         if (self->string_size > pos) self->string_size = pos;
+        self->pos = self->string_size;
 
         Py_INCREF(Py_None);
         return Py_None;