operation: avoid state change from final state

The internal operation_set_state function already returns early if the
new state is the same as the existing state. The attached patch extends
this to return early if already in a finalised (done/cancelled) state,
i.e. blocks attempts to re-finalise into a different state.

This helps avoid unlinking more than once (or crashing on ref count
assertion).

I was not certain whether an assertion would be a better alternative -
with such a crash helping highlight usage problems...

The situation that lead to this was the thought of someone stupidly
trying to pa_operation_cancel() a callback within the callback
execution itself, while designing a solution for a memory leak related
to cancellation within my Rust binding. While no-one should do such a
thing, if they did, they'd either trip up a ref count assertion, or the
operation would be unlinked twice, which would be bad. It's a simple
thing to catch and mitigate, and could prove to be a useful
bulletproofing measure for this function in general.

src/pulse/operation.c

@@ -102,6 +102,9 @@ static void operation_set_state(pa_operation *o, pa_operation_state_t st) {
     if (st == o->state)
         return;
 
+    if ((o->state == PA_OPERATION_DONE) || (o->state == PA_OPERATION_CANCELED))
+        return;
+
     pa_operation_ref(o);
 
     o->state = st;