SCC: Handle non-matched SCC codes in SccWord.get_channel

[atsampson]

If is_code returns True but _find_code doesn't match the code, get_channel would crash rather than returning None.

(This was another case of a somewhat-mangled CC stream from a noisy LaserDisc via ld-decode; the invalid words were 1f38, 1d00, 1432, 1000.)

Closes #428

[palemieux]

Feels like is_code() should return False if self.code is None.

[atsampson]

That was my first thought, but it's used in _find_code which is called from the constructor. Maybe _find_code should just try and match regardless?

[palemieux]

I see now, and agree that your patch fixes the immediate issue.

I see a broader issue with the semantics of is_code() and get_code() but probably should be fixed as a separate issue.

Can you add a test case that covers the issue that triggered the PR, and then we can merge.

Thanks!

[atsampson]

I've added a unit test for SccWord.get_channel, and another that tests that all the methods return something for all possible codes.