Fix Coverity Scan CID 300455 (Using uninitialized value msg when calling...

Fix Coverity Scan CID 300455 (Using uninitialized value msg when calling flexran_agent_mac_sf_trigger.)

openair2/ENB_APP/CONTROL_MODULES/MAC/flexran_agent_mac.c

@@ -1346,7 +1346,7 @@ void flexran_agent_send_sr_info(mid_t mod_id) {
 
 void flexran_agent_send_sf_trigger(mid_t mod_id) {
   int size;
-  Protocol__FlexranMessage *msg;
+  Protocol__FlexranMessage *msg = NULL;
   void *data;
   int priority = 0;
   err_code_t err_code;