qapi: Fix regression with '-netdev help'

Commit e36c714e causes 'qemu -netdev help' to dump core, because the
call to visit_end_union() is no longer conditional on whether *obj was
allocated.

Reported by Marc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1444861825-19256-1-git-send-email-eblake@redhat.com>
[Commit message tweaked to say 'help' instead of '?']
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi-visit.py

@@ -301,7 +301,9 @@ def gen_visit_union(name, base, variants):
 out_obj:
     error_propagate(errp, err);
     err = NULL;
-    visit_end_union(v, !!(*obj)->data, &err);
+    if (*obj) {
+        visit_end_union(v, !!(*obj)->data, &err);
+    }
     error_propagate(errp, err);
     err = NULL;
     visit_end_struct(v, &err);