tools: ynl: avoid print_field when there is no reply

When request a none support device operation, there will be no reply.
In this case, the len(desc) check will always be true, causing print_field
to enter an infinite loop and crash the program. Example reproducer:

# ethtool.py -c veth0

To fix this, return immediately if there is no reply.

Fixes: f3d07b02b2b8 ("tools: ynl: ethtool testing tool")
Signed-off-by: Hangbin Liu <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/net/ynl/pyynl/ethtool.py

···
     Pretty-print a set of fields from the reply. desc specifies the
     fields and the optional type (bool/yn).
     """
+    if not reply:
+        return
+
     if len(desc) == 0:
         return print_field(reply, *zip(reply.keys(), reply.keys()))