KeyError address not present while loading dump

[Martin Packman]

From a dump generated on a win64 machine, there are various objects that reference things that don't exist and generate an error during loading.

This can minimally be reproduced with:

>>> from meliae.loader import load
>>> load(['''{"address": 55709768, "type": "module", "size": 48, "name": "_random", "refs": [55131496]}'''])
loaded line 0, 1 objs, 0.0 / 0.0 MiB read in 0.0s
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Dev\python\2.7\lib\site-packages\meliae\loader.py", line 548, in load
    if not manager.collapse_instance_dicts():
  File "C:\Dev\python\2.7\lib\site-packages\meliae\loader.py", line 416, in collapse_instance_dicts
    (dict_obj,) = obj
  File "_loader.pyx", line 501, in meliae._loader._MemObjectProxy.__getitem__ (meliae\_loader.c:4193)
  File "_loader.pyx", line 497, in meliae._loader._MemObjectProxy.__getitem__ (meliae\_loader.c:4146)
  File "_loader.pyx", line 778, in meliae._loader.MemObjectCollection.__getitem__ (meliae\_loader.c:7246)
KeyError: 'address 55131496 not present'

There's a comment about this problem at the call site, which addressing allows the dump to load at least:

      # TODO: What to do if the object isn't present? I think returning a
      # 'no-such-object' proxy would be nicer than returning nothing