can_datahandler returns an unsigned value that represent the
number of bytes actually bufffered, but the call to iob_tryadd_queue
returns a negative value when it fails so that value can't be used as
can_datahandler return value because it is interpreted as unsigned so
a big number is wrongly returned when iob_tryadd_queue fails.
The fix is just setting the return value to zero when iob_tryadd_queue
fails, reflecting the actual meaning that no bytes have been buffered.
52dbfda355