PYTHON-4323 Add regression test for out-of-bounds read when decoding invalid bson

[ShaneHarvey]

https://jira.mongodb.org/browse/PYTHON-4323

Here's how this test fails on pymongo 4.6.0:

FAIL: test_bad_code (__main__.TestBSON.test_bad_code)
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bson.errors.InvalidBSON: Detected unknown BSON type b'\x00' for fieldname '00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'. Are you using the latest driver version?

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/shane/git/mongo-python-driver/test/test_bson.py", line 516, in test_bad_code
    with self.assertRaisesRegex(InvalidBSON, "invalid") as ctx:
AssertionError: "invalid" does not match "Detected unknown BSON type b'\x00' for fieldname '00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'. Are you using the latest driver version?"