fix: skip zero-length ISO OUT packets in audio read thread

[Nictrla]

A zero-length isochronous OUT transfer (actual_length == 0) carries no audio data, but the read thread still recorded it as a frame: it wrote frame_length = 0 into the slot at transfer_pos, advanced transfer_pos and fired the frame_done callback.

Because the frame FIFO uses frame_length == 0 as the sentinel for an empty slot, this made the slot indistinguishable from a free one. The consumer then read at access_pos, saw frame_length == 0, reported it as an underflow (UX_BUFFER_OVERFLOW) and did not advance access_pos. As a result transfer_pos moved forward while access_pos stayed put, permanently desyncing the producer/consumer positions.

Skip zero-length packets entirely: leave transfer_pos in place, do not fire the callback, and re-issue the transfer at the same slot. This preserves the "frame_length == 0 means empty slot" invariant and keeps the transfer and access positions in sync.

[fdesbiens]

Hi @Nictrla.

Thank you for this contribution!

@rahmanih: Can you review, please?

[rahmanih]

usbx/common/usbx_device_classes/src/ux_device_class_audio_read_task_function.c

Line 152 in ea1c406

actual_length = transfer -> ux_slave_transfer_request_actual_length;

fix to be applied here too.