- Use e212 routine for IMSI.
- Add top_tree to function calls to avoid a crash.
- Set a21_prefs_initialized.

Change-Id: I193ae87189dda9b3806f43c9be3916f1adf7bda2
Reviewed-on: https://code.wireshark.org/review/3902
Reviewed-by: Anders Broman <a.broman58@gmail.com>