FrameVars seems to assume that Frame.block can return None if there is
no block. However, it actually throws an exception.
I saw this bug while developing a frame filter, but unfortunately I
don't know how to reproduce it. It seems to me that the SAL tests in
_is_limited_frame should exclude the bad cases; and in my attempts to
write a test they do.
Nevertheless I think the fix is reasonably obvious and ought to go in.