qapi: Catch and reject flat union branch of array type

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/scripts/qapi.py b/scripts/qapi.py
index 8f23267..06d7fc2 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -580,7 +580,7 @@
         # Each value must name a known type; furthermore, in flat unions,
         # branches must be a struct with no overlapping member names
         check_type(expr_info, "Member '%s' of union '%s'" % (key, name),
-                   value, allow_array=True, allow_metas=allow_metas)
+                   value, allow_array=not base, allow_metas=allow_metas)
         if base:
             branch_struct = find_struct(value)
             assert branch_struct