rbus: fixup reverse null check in rbusEvent_SubscribeEx

[svc-rdkeportal01]

Fix REVERSE_INULL in rbusEvent_SubscribeEx

Issues Fixed

• Coverity CID 140: REVERSE_INULL in src/rbus/rbus.c at lines 5173, 5179, 5181

Root Cause

The code was dereferencing response pointer at lines 5173 and 5179 before checking if it's NULL at line 5181. This is a logic error - if response was NULL, the code would crash before reaching the NULL check.

Problematic Code:

if(publishOnSubscribe)
{
    rbusMessage_GetInt32(response, &initial_value);  // Line 5173: Deref!
    // ...
}
rbusMessage_GetInt32(response, &subscriptionId);  // Line 5179: Deref again!
subInternal->subscriptionId = subscriptionId;
if(response)  // Line 5181: Check NULL (too late!)
    rbusMessage_Release(response);

Changes Made

Added early NULL check before any dereference operations, with proper cleanup and enhanced error handling:

Fixed Code:

/* Validate response pointer before any dereference operations */
if(!response)
{
    RBUSLOG_ERROR("%s subscribe failed: null response received from provider", eventName);
    
    /* Cleanup: Remove the subscription entry that was just added */
    HANDLE_EVENTSUBS_MUTEX_LOCK(handle);
    rtVector_RemoveItem(handleInfo->eventSubs, subInternal, rbusEventSubscription_compare);
    HANDLE_EVENTSUBS_MUTEX_UNLOCK(handle);
    
    /* Free allocated subscription structure */
    free(subInternal);
    
    return RBUS_ERROR_INVALID_RESPONSE;
}

/* Safe to dereference response now */
if(publishOnSubscribe)
{
    int result = rbusMessage_GetInt32(response, &initial_value);
    if(result == RBUS_ERROR_SUCCESS && initial_value)
    {
        _master_event_callback_handler(NULL, eventName, response, userData);
    }
    else if(result != RBUS_ERROR_SUCCESS)
    {
        RBUSLOG_WARN("%s subscribe: failed to get initial value from response (error=%d)", 
                    eventName, result);
    }
}

/* Extract subscription ID from response */
int result = rbusMessage_GetInt32(response, &subscriptionId);
if(result == RBUS_ERROR_SUCCESS)
{
    subInternal->subscriptionId = subscriptionId;
    RBUSLOG_DEBUG("%s subscribe: assigned subscriptionId=%d", eventName, subscriptionId);
}
else
{
    RBUSLOG_WARN("%s subscribe: failed to get subscription ID from response (error=%d), using default", 
                eventName, result);
    subInternal->subscriptionId = 0;
}

rbusMessage_Release(response);

Why This Fix is Correct

1. Early NULL Check - Validates response BEFORE any dereference
2. Proper Cleanup - Removes subscription entry and frees memory on error
3. Error Handling - Returns RBUS_ERROR_INVALID_RESPONSE for NULL response
4. Enhanced Validation - Checks return values of rbusMessage_GetInt32()
5. Graceful Degradation - Uses default subscriptionId=0 if extraction fails
6. Comprehensive Logging - Debug/warn messages for troubleshooting
7. Thread-Safe - Proper mutex handling in cleanup path
8. Memory-Safe - No memory leaks on error path

• Changes: +43 lines, -6 lines