packet-a21.c: Fix a bug; Do some other misc cleanup.

- expert_...() and external dissectors shouldn't be called under 'if (tree)';
- Remove (what appear to be) some unneeded casts;
- (Misc cleanup)

Change-Id: I5ced313d3342125b918e179529b12d2f3bc4d87a
Reviewed-on: https://code.wireshark.org/review/3830
Reviewed-by: Anders Broman <a.broman58@gmail.com>