Crashes with bson error when oops is corrupted (zero bytes size)

[Roman Yepishev]

At the moment if oops is 0 bytes long (either .oops or decompressed from .bz) the reader fails with:

Tests running...
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ERROR: oops_datedir_repo.tests.test_serializer.TestParsing.test_ioerror_on_empty_oops
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Traceback (most recent call last):
  File "oops_datedir_repo/tests/test_serializer.py", line 73, in test_ioerror_on_empty_oops
    self.assertRaises(IOError, read, source_file)
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/testtools-0.9.12-py2.6.egg/testtools/testcase.py", line 388, in assertRaises
    self.assertThat(lambda: callableObj(*args, **kwargs), matcher)
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/testtools-0.9.12-py2.6.egg/testtools/testcase.py", line 400, in assertThat
    mismatch = matcher.match(matchee)
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/testtools-0.9.12-py2.6.egg/testtools/matchers.py", line 745, in match
    mismatch = self.exception_matcher.match(exc_info)
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/testtools-0.9.12-py2.6.egg/testtools/matchers.py", line 495, in match
    mismatch = matcher.match(matchee)
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/testtools-0.9.12-py2.6.egg/testtools/testcase.py", line 380, in match
    reraise(*matchee)
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/testtools-0.9.12-py2.6.egg/testtools/matchers.py", line 738, in match
    result = matchee()
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/testtools-0.9.12-py2.6.egg/testtools/testcase.py", line 388, in <lambda>
    self.assertThat(lambda: callableObj(*args, **kwargs), matcher)
  File "oops_datedir_repo/serializer.py", line 52, in read
    return serializer_bson.read(StringIO(content))
  File "oops_datedir_repo/serializer_bson.py", line 64, in read
    report = bson.loads(fp.read())
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/bson-0.3.2-py2.6.egg/bson/__init__.py", line 75, in loads
    return decode_document(data, 0)[1]
  File "/home/rtg/Projects/python-oops-datedir-repo/eggs/bson-0.3.2-py2.6.egg/bson/codec.py", line 235, in decode_document
    length = struct.unpack("<i", data[base:base + 4])[0]
error: unpack requires a string argument of length 4