selftests: net: devlink_port_split: check devlink returned an element before dereferencing it

And thus avoid a Python stacktrace:

~/linux/tools/testing/selftests/net$ ./devlink_port_split.py
Traceback (most recent call last):
File "/home/linux/tools/testing/selftests/net/./devlink_port_split.py",
line 277, in <module> main()
File "/home/linux/tools/testing/selftests/net/./devlink_port_split.py",
line 242, in main
dev = list(devs.keys())[0]
IndexError: list index out of range

Signed-off-by: Paolo Pisati <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/devlink_port_split.py

···
         assert stderr == ""
 
         devs = json.loads(stdout)['dev']
+        if len(devs.keys()) == 0:
+            print("no devlink device found")
+            sys.exit(1)
         dev = list(devs.keys())[0]
 
     cmd = "devlink dev show %s" % dev