[portstat] Fixed delete when no data saved

[msosyak]

- What I did
Added logic to avoid exception on delete when no data saved

- How I did it
Added isdir check before delete logic

- How to verify it
run portstat -D twice

- Previous command output (if the output of a command-line utility has changed)

admin@dut:~$ portstat -D
admin@dut:~$ portstat -D
Traceback (most recent call last):
  File "/usr/bin/portstat", line 372, in <module>
    main()
  File "/usr/bin/portstat", line 297, in main
    for file in os.listdir(cnstat_dir):
OSError: [Errno 2] No such file or directory: '/tmp/portstat-1000'
admin@dut:~$

- New command output (if the output of a command-line utility has changed)

admin@dut:~$ portstat -D
admin@dut:~$ portstat -D
admin@dut:~$ 

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please

[msosyak]

retest this please