Feedback Agreement, and a PR guarding two crash paths (#5104)

[Przymrozek]

Hello,

I am Bartłomiej Mróz, working on a containerised Higher-Order Ambisonics and 360 video livestream toolchain that uses dash.js for playback. My signed Feedback Agreement is attached; I am submitting as an individual.

I have opened #5104, which guards three unguarded reads that throw on input the player can legitimately be handed: SegmentTimeline without S elements in TimelineSegmentsGetter, and the assumption of ISOBMFF boxes in BoxParser.getSamplesInfo and the CEA-608 path in TextSourceBuffer, which WebM segments do not satisfy. I ran into this failure class while streaming WebM live, and had been carrying local patches for it since June. Unit tests are included, each failing on development and passing with the change.

The timeline crash was reported before as #3513 and #2708 but closed without the read gaining a guard; I could not find anything covering the non-ISOBMFF path. If anyone is already working in that area I am happy to step aside or rework the patch, and equally happy to split it into two PRs if that is easier to review.

Kind regards, 

Bart