Properly initialize the thread local objects

Instead of doing assignment. Assignment will probably reinitialize every
cycle, and it also didn't work on one person's computer.

src/common/communication/vst3.h

@@ -212,11 +212,8 @@ class Vst3MessageHandler : public AdHocSocketHandler<Thread> {
         // process data, since that's the only object where allocations can
         // happen. The other objects we're storing here are very small, so the
         // extra wasted memory shouldn't matter much.
-        // FIXME: This of course doesn't work. The variant will have been
-        //        reconstructed when the next process cycle hits, so we don't
-        //        gain anything from this.
-        thread_local std::optional<Request> persistent_object =
-            persistent_buffers ? std::make_optional<Request>() : std::nullopt;
+        thread_local std::optional<Request> persistent_object(
+            persistent_buffers ? std::make_optional<Request>() : std::nullopt);
 
         // Reading, processing, and writing back the response for the requests
         // we receive works in the same way regardless of which socket we're