Check for buffered_caption being None in SCC reader

[atsampson]

This fixes a problem found when parsing CC data from a noisy LaserDisc: if a signal dropout corrupts the preamble for a caption, the SCC reader shouldn't crash when text is seen for that caption.

[palemieux]

It looks like the code throws in other places where no caption is initialized:

• ttconv/src/main/python/ttconv/scc/reader.py

  Line 300 in 0da1242

  if processed_caption is None or processed_caption.get_current_text() is None:

• ttconv/src/main/python/ttconv/scc/reader.py

  Line 244 in 0da1242

  if processed_caption is None:

@atsampson Is your proposal to basically ignore 608/708 codes until the next caption?

@valnoel Any opinion?

[valnoel]

Hi @atsampson @palemieux !

I've completed the @atsampson suggestion by skipping captions parsing in case of no PAC is detected from the SCC stream (i.e. when the cursor position is not specified). I've added a unit test as well.

See #385

[GRDazzle]

I have run into this problem as well. I can confirm the PR change fix the issue for me.

[GRDazzle]

This is tested and verified with my own test case.

[palemieux]

Mooted by #395