[S1AP) Wrong tree used
s1ap.cnf:547:14: error: variable 'subtree' set but not used

Change-Id: I84a345709b1ef9688f3f1a4e876a75dd575c1c2e
Reviewed-on: https://code.wireshark.org/review/5299
Reviewed-by: Anders Broman <a.broman58@gmail.com>