threads: fix race warnings for client-side debouncing

Client-side thread debouncing happens in a different goroutine. Any
function or variable that is called or accessed by this goroutine should
be protected from a concurrent access.

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

widgets/msglist.go

@@ -88,7 +88,7 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 	)
 
 	if store.ThreadedView() {
-		threads := store.Threads
+		threads := store.Threads()
 		counter := len(store.Uids())
 
 		for i := len(threads) - 1; i >= 0; i-- {