traceio.stp example: check for negative $return

traceio.stp example: check for negative $return

The I/O reads and writes should only be counted when $return is
nonnegative. A negative $result occurs if an error was met (e.g.
EAGAIN = -11).

We could have instead here used the bytes_read/written variables
provided by the tapset, but this example demonstrates the use of
local vars and is also consistent with the disktop.stp example.