PR #52 introduced a code path to `ghostify` that calls PyErr_Clear()
with the intent to avoid propagating AttributeErrors for slots.

However, if there is an error (like a POSKeyError) raised by
jar.setstate(), then `unghostify` will call ghostify with an error
pending. If the object had slots that weren't set and the
AttributeError was cleared, so was the pending error from setstate. So
when `ghostify` returned NULL that got propagated up to the
interpreter which finds no exception and so raises `SystemError: error
return without exception set`.

This commit makes `unghostify` save and restore the exception state
around the call to PyErr_Clear.