packet-a21.c: Fix a bug;

- expert_...() and external dissectors shouldn't be called under 'if (tree)';

Change-Id: I11dc61d11be6179a22d63a72847e7c76a9ff7a5a
Reviewed-on: https://code.wireshark.org/review/3835
Reviewed-by: Bill Meier <wmeier@newsguy.com>