qtdemux: Fix av1C parsing

Edward Hervey requested to merge bilboed/gstreamer:qtdemux-av1c into main

This is a regression introduced by !3882 (merged)

The av1c codec configuration parsing would always fail due to an off-by-one error, the content of an atom starting at offset 8 (i.e. the 9th byte) and not 9 (the 10th byte).

Also introduce a break in order to not get stray warnings

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