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: NPaolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/devlink_port_split.py

@@ -239,6 +239,9 @@ def main(cmdline=None):
         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