client: do not crash when overriding remote command as method

Do not crash during API initialization when overriding remote command that
is not a method with MethodOverride.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipaclient/frontend.py

@@ -55,11 +55,17 @@ class CommandOverride(Command):
 class MethodOverride(CommandOverride, Method):
     @property
     def obj_name(self):
-        return self.next.obj_name
+        try:
+            return self.next.obj_name
+        except AttributeError:
+            return None
 
     @property
     def attr_name(self):
-        return self.next.attr_name
+        try:
+            return self.next.attr_name
+        except AttributeError:
+            return None
 
     @property
     def obj(self):