When attempting to produce a repr for local variables, attempt to cope
better with trashed memory:

- Use "<invalid object at 0x1234>" as the repr of a `PyObject*` pointing
  directly at invalid memory, or one whose `ob_type` points at invalid
  memory.
- Use "<list object at 0x1234>" as, for instance, the repr of a list containing an
  invalid `ob_items` pointer. More generally, use this for any object
  where we can determine the type, but generating the repr according to
  that type's rules fails due to accessing unmapped memory.

Test this with some ctypes abuse.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>