uvstream: avoid LIBUSB_TRANSFER_OVERFLOW
uvstream: avoid LIBUSB_TRANSFER_OVERFLOW

Fix for https://github.com/AravisProject/aravis/issues/126

If arv_stream_pop_input_buffer returns NULL, the thread stays in the
state where it expects a leader, while the device sends image data.

Because the payload data is larger than the requested size for the
leader, libusb then reports LIBUSB_TRANSFER_OVERFLOW, which, according
to its documentation, should be avoided.

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