There are situations possible when both exc_traceback and frame objects are
garbage-collected, but frame's f_locals remains not collected because e.g. it
was explicitly added to somewhere. We cannot detect such cases (dicts are not
listed in referrers).

So if nothing helped, as a last resort, unpin pybuf from its original
memory and make it point to zero-sized NULL.

In general this is not strictly correct to do as other buffers &
memoryview objects created from pybuf, copy its pointer on
initialization and thus pybuf unpinning won't adjust them.

However we require BigFile implementations to make sure not to use
such-created objects, if any, after return from loadblk().

Finally fixes nexedi/wendelin.core#7