libc: Fix an invalid __vm_obj_free() in case no atexit handlers are registered.

dios/libc/stdlib/atexit.c

@@ -69,7 +69,10 @@ int __cxa_finalize( void *dso_handle )
             entry->func = NULL;
         }
     }
-    __vm_obj_free( atexit_entries );
+
+    if ( atexit_entries )
+        __vm_obj_free( atexit_entries );
+
     // ToDo: Remove invalid entries?
     return 0;
 }