qtdemux: Skip zero-sized boxes instead of stopping to look at further boxes

qtdemux: Skip zero-sized boxes instead of stopping to look at further boxes

A zero-sized box is not really a problem and can be skipped to look at any
possibly following ones.

BMD ATEM devices specifically write a zero-sized bmdc box in the sample
description, followed by the avcC box in case of h264. Previously the avcC box
would simply not be read at all and the file would be unplayable.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7565>