rbus: fix NULL pointer dereference in _subscribe_async_callback_handler

[svc-rdkeportal01]

Issue Fixed

Coverity Defect: REVERSE_INULL
CWE: CWE-476 (NULL Pointer Dereference)
Severity: High
Function: _subscribe_async_callback_handler
File: src/rbus/rbus.c

Root Cause

The function _subscribe_async_callback_handler has a classic REVERSE_INULL defect:

1. Line 1367: Dereferences subscription->asyncHandler
2. Line 1368: Checks if subscription is NULL

The NULL check comes AFTER the dereference, making it useless. If subscription is NULL, the program will crash at line 1367 before reaching the NULL check.

Changes Made

Before (Buggy):

subscription->asyncHandler(subscription->handle, subscription, error);  // ❌ Dereference first
if(subscription)  // ❌ Check second - too late!
{
    // ... rest of code
}

After (Fixed):

if(subscription)  // ✅ Check first
{
    subscription->asyncHandler(subscription->handle, subscription, error);  // ✅ Dereference second - safe!
    // ... rest of code
}
else
{
    RBUSLOG_ERROR("Null subscription pointer in _subscribe_async_callback_handler");
}

Why This Fix is Correct

1. Prevents crash - NULL check happens before dereference
2. Proper error handling - Logs error when subscription is NULL
3. Simple fix - Just reorder the code, no complex logic
4. Maintains functionality - All existing logic preserved, just safer

Testing

• Verified fix compiles without errors
• Checked that NULL case is properly handled
• Confirmed error logging is in place