qapi: Fix crash on missing enum member name

qapi: Fix crash on missing enum member name

New test case enum-dict-no-name.json crashes:

    $ python3 scripts/qapi-gen.py tests/qapi-schema/enum-dict-no-name.json
    Traceback (most recent call last):
    [...]
      File "/work/armbru/qemu/scripts/qapi/expr.py", line 458, in check_enum
	member_name = member['name']
    KeyError: 'name'

Root cause: we try to retrieve member 'name' before we check for
missing members.  With that fixed, we get the expected error "'data'
member misses key 'name'".

Fixes: 0825f62c842f2c07c5471391c6d7fd3f4fe83732
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210616072121.626431-1-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>