Linux: Don't free() URBs prematurely on hardware error; fix #54

Linux: Don't free() URBs prematurely on hardware error; fix #54

When an URB in a multi-URB transfer had an error, handle_bulk_completion()
would free all URBs and segfault once the next one completed, instead of
cancelling the remaining URBs and cleaning up as usual.

This is basically the patch from the ticket, plus some restructuring
for increased readability of the function.

Many thanks to Brian Shirley and National Instruments for finding and
fixing this!