call tbf_unlink_pdch() from gprs_rlcmac_tbf destructor

Change-Id: Iadec328774291e09f45850e0514653ac679c3451
diff --git a/src/tbf.cpp b/src/tbf.cpp
index fcad879..56f3d76 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -164,6 +164,7 @@
 
 gprs_rlcmac_tbf::~gprs_rlcmac_tbf()
 {
+	tbf_unlink_pdch(this);
 	osmo_fsm_inst_free(state_fsm.fi);
 	state_fsm.fi = NULL;
 }