SF patch 514641 (Naofumi Honda) - Negative ob_size of LongObjects

Due to the bizarre definition of _PyLong_Copy(), creating an instance
of a subclass of long with a negative value could cause core dumps
later on.  Unfortunately it looks like the behavior of _PyLong_Copy()
is quite intentional, so the fix is more work than feels comfortable.

This fix is almost, but not quite, the code that Naofumi Honda added;
in addition, I added a test case.