scripts: remove extra NUL at the end of iperf3 pidfile

There is an extra NUL(\0) byte at the end of pidfile
due to iperf3 bug. When stop iperf3 server by pidfile
will raise warning:
command substitution: ignored null byte in input.

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>

Change-Id: I51228fa20304087d69e9d44c0a4238a3c21e1b7e