Consistent ControlList freezing -- fixes #7419

AudioRegion::set_fade_in() freezes the original ControlList, then
assigns a new one and thaws that.
Frozen state needs to be retained during assignment.

Related: The overloaded assignment operator in AutomationList
performed duplicate signal emission and didn't freeze the list.

libs/evoral/src/ControlList.cpp

@@ -152,7 +152,8 @@ ControlList&
 ControlList::operator= (const ControlList& other)
 {
 	if (this != &other) {
-		_frozen = 0;
+		/* list should be frozen before assignment */
+		assert (_frozen > 0);
 		_changed_when_thawed = false;
 		_sort_pending = false;
 
@@ -166,7 +167,6 @@ ControlList::operator= (const ControlList& other)
 		_desc = other._desc;
 		_interpolation = other._interpolation;
 
-		// XXX copy_events() emits Dirty, but this is just assignment copy/construction
 		copy_events (other);
 	}