Senseless to initialize value to NULL; better (and clearer) to <code>
const void *value = EvalContextVariableControlCommonGet(ctx, COMMON_CONTROL_LASTSEEN_EXPIRE_AFTER);
if (value) { ... }
</code>
The whole "assignment inside a conditional" thing only really makes sense in the condition of a while loop; it's sometimes worth it in a compound conditional for an if, as a way to achieve brevity, but seldom in a simple one.