Fixed possible crash and removed trace from greentea.

Author: Teppo Järvelin

features/cellular/TESTS/socket/udp/template_mbed_app.json.txt

@@ -31,7 +31,7 @@
             "ppp-cell-iface.apn-lookup": false,
             "cellular.use-apn-lookup": false,
             "target.features_add": ["LWIP", "COMMON_PAL"],
-            "mbed-trace.enable": true,
+            "mbed-trace.enable": false,
             "lwip.ipv4-enabled": true,
             "lwip.ipv6-enabled": true,
             "lwip.tcp-enabled": false,

features/cellular/easy_cellular/CellularConnectionFSM.cpp

@@ -75,8 +75,10 @@ CellularConnectionFSM::~CellularConnectionFSM()
 void CellularConnectionFSM::stop()
 {
     tr_info("CellularConnectionUtil::stop");
-    _cellularDevice->close_power();
-    _cellularDevice->close_network();
+    if (_cellularDevice) {
+        _cellularDevice->close_power();
+        _cellularDevice->close_network();
+    }
     if (_queue_thread) {
         _queue_thread->terminate();
         delete _queue_thread;