Sound: Fix asymmetrical mutex lock/unlock logic

Started to happen after recent fix for T72632.

Was caused by runtime fields backup doing an early exit in the case the
given ID was never expanded by the Copy-on-Write mechanism, but it was
not done int the backup restore function (since it was not possible to
know "locally").

Now both init() and restore() will do an early exit when the ID had
nothing to be backed up.

source/blender/depsgraph/intern/eval/deg_eval_runtime_backup.cc

@@ -32,7 +32,8 @@
 namespace DEG {
 
 RuntimeBackup::RuntimeBackup(const Depsgraph *depsgraph)
-    : animation_backup(depsgraph),
+    : have_backup(nullptr),
+      animation_backup(depsgraph),
       scene_backup(depsgraph),
       sound_backup(depsgraph),
       object_backup(depsgraph),
@@ -48,6 +49,7 @@ void RuntimeBackup::init_from_id(ID *id)
   if (!deg_copy_on_write_is_expanded(id)) {
     return;
   }
+  have_backup = true;
 
   animation_backup.init_from_id(id);
 
@@ -83,6 +85,10 @@ void RuntimeBackup::init_from_id(ID *id)
 
 void RuntimeBackup::restore_to_id(ID *id)
 {
+  if (!have_backup) {
+    return;
+  }
+
   animation_backup.restore_to_id(id);
 
   const ID_Type id_type = GS(id->name);

source/blender/depsgraph/intern/eval/deg_eval_runtime_backup.h

@@ -46,6 +46,17 @@ class RuntimeBackup {
   /* Restore fields to the given ID. */
   void restore_to_id(ID *id);
 
+  /* Denotes whether init_from_id did put anything into the backup storage.
+   * This will not be the case when init_from_id() is called for an ID which has never been
+   * copied-on-write. In this case there is no need to backup or restore anything.
+   *
+   * It also allows to have restore() logic to be symmetrical to init() without need to worry
+   * that init() might not have happenned.
+   *
+   * In practice this is used by audio system to lock audio while scene is going through
+   * copy-on-write mechanism. */
+  bool have_backup;
+
   AnimationBackup animation_backup;
   SceneBackup scene_backup;
   SoundBackup sound_backup;