Native: don't dispatch release for null Obj-C refs to the main thread

Currently, if a Kotlin object happens to have null for an associated
object but FLAGS_RELEASE_ON_MAIN_QUEUE flag set, GC will dispatch
Kotlin_ObjCExport_releaseAssociatedObject(null) to the main thread
anyway.

This couldn't happen before, but can now, with disposeObjCObject.
The commit prevents this, by moving the null check out from
Kotlin_ObjCExport_releaseAssociatedObject to call sites.

^KT-59134

kotlin-native/runtime/src/legacymm/cpp/Memory.cpp

@@ -3085,7 +3085,9 @@ void ObjHeader::destroyMetaObject(ObjHeader* object) {
   }
 
 #ifdef KONAN_OBJC_INTEROP
-  Kotlin_ObjCExport_releaseAssociatedObject(meta->associatedObject_);
+  if (void* associatedObject = meta->associatedObject_) {
+    Kotlin_ObjCExport_releaseAssociatedObject(associatedObject);
+  }
 #endif
 
   std_support::allocator_delete(objectAllocator, meta);

kotlin-native/runtime/src/main/cpp/ObjCExport.mm

@@ -107,11 +107,10 @@ ALWAYS_INLINE void send_releaseAsAssociatedObject(void* associatedObject) {
 } // namespace
 
 extern "C" ALWAYS_INLINE void Kotlin_ObjCExport_releaseAssociatedObject(void* associatedObject) {
-  if (associatedObject != nullptr) {
-    // May already be in the native state if was scheduled on the main queue.
-    NativeOrUnregisteredThreadGuard guard(/*reentrant=*/ true);
-    send_releaseAsAssociatedObject(associatedObject);
-  }
+  RuntimeAssert(associatedObject != nullptr, "Kotlin_ObjCExport_releaseAssociatedObject(nullptr)");
+  // May already be in the native state if was scheduled on the main queue.
+  NativeOrUnregisteredThreadGuard guard(/*reentrant=*/ true);
+  send_releaseAsAssociatedObject(associatedObject);
 }
 
 extern "C" id Kotlin_ObjCExport_convertUnitToRetained(ObjHeader* unitInstance) {

kotlin-native/runtime/src/mm/cpp/ExtraObjectData.cpp

@@ -60,14 +60,16 @@ void mm::ExtraObjectData::Uninstall() noexcept {
             this);
 
 #ifdef KONAN_OBJC_INTEROP
-    if (getFlag(FLAGS_RELEASE_ON_MAIN_QUEUE) && isMainQueueProcessorAvailable()) {
-        runOnMainQueue(associatedObject_, [](void* obj) {
-            Kotlin_ObjCExport_releaseAssociatedObject(obj);
-        });
-    } else {
-        Kotlin_ObjCExport_releaseAssociatedObject(associatedObject_);
+    if (void* associatedObject = associatedObject_) {
+        if (getFlag(FLAGS_RELEASE_ON_MAIN_QUEUE) && isMainQueueProcessorAvailable()) {
+            runOnMainQueue(associatedObject, [](void* obj) {
+                Kotlin_ObjCExport_releaseAssociatedObject(obj);
+            });
+        } else {
+            Kotlin_ObjCExport_releaseAssociatedObject(associatedObject);
+        }
+        associatedObject_ = nullptr;
     }
-    associatedObject_ = nullptr;
 #endif
 }