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.

Lib/test/test_descr.py

@@ -1748,6 +1748,10 @@ def inherits():
     verify((a + 0).__class__ is long)
     verify((0 + a).__class__ is long)
 
+    # Check that negative clones don't segfault
+    a = longclone(-1)
+    vereq(a.__dict__, {})
+
     class precfloat(float):
         __slots__ = ['prec']
         def __init__(self, value=0.0, prec=12):