Cellular: fix possible crash in state machine

_sim_pin was changed to pointer from array and length was checked with
strlen. If _sim_pin was null it caused crash. Fix by checking _sim_pin against NULL.
Power class could have been called without checking if power is NULL. Fix by checking
that power class is not null.
Fix state machine to return correct states when queried.

Author: Teppo Järvelin

features/cellular/framework/device/CellularStateMachine.cpp

@@ -153,7 +153,7 @@ bool CellularStateMachine::open_sim()
     }
 
     if (state == CellularDevice::SimStatePinNeeded) {
-        if (strlen(_sim_pin)) {
+        if (_sim_pin) {
             tr_info("Entering PIN to open SIM");
             _cb_data.error = _cellularDevice.set_pin(_sim_pin);
             if (_cb_data.error) {
@@ -428,13 +428,13 @@ void CellularStateMachine::state_sim_pin()
         }
 
         if (_network->is_active_context()) { // check if context was already activated
-            tr_debug("ACTIVE CONTEXT FOUND, skip registering.");
+            tr_debug("Active context found.");
             _network_status |= ACTIVE_PDP_CONTEXT;
         }
         CellularNetwork::AttachStatus status; // check if modem is already attached to a network
         if (_network->get_attach(status) == NSAPI_ERROR_OK && status == CellularNetwork::Attached) {
             _network_status |= ATTACHED_TO_NETWORK;
-            tr_debug("DEVICE IS ALREADY ATTACHED TO NETWORK, skip registering and attach.");
+            tr_debug("Cellular already attached.");
         }
         if (_plmn) {
             enter_to_state(STATE_MANUAL_REGISTERING_NETWORK);

features/cellular/framework/device/CellularStateMachine.h

@@ -42,6 +42,7 @@ class CellularStateMachine {
     // friend of CellularDevice so that it's the only way to close/delete this class.
     friend class CellularDevice;
     friend class AT_CellularDevice;
+    friend class UT_CellularStateMachine; // for unit tests
     /** Constructor
      *
      * @param device    reference to CellularDevice
@@ -98,7 +99,7 @@ class CellularStateMachine {
      *  @param timeout      timeout array using seconds
      *  @param array_len    length of the array
      */
-    void set_retry_timeout_array(uint16_t timeout[], int array_len);
+    void set_retry_timeout_array(uint16_t *timeout, int array_len);
 
     /** Sets the operator plmn which is used when registering to a network specified by plmn. If plmn is not set then automatic
      *  registering is used when registering to a cellular network. Does not start any operations.