msgstore: fetch missing headers in visual mode

fetches missing headers when in visual selection mode. This prevents
large archive operations from panicking due to a nil pointer
dereference.

The archive command will return an error to the ui when a nil message is
encountered to signal that the message store is not ready yet.

Signed-off-by: Koni Marti <koni.marti@gmail.com>

lib/msgstore.go

@@ -523,6 +523,13 @@ func (store *MessageStore) updateVisual() {
 	for _, uid := range visUids {
 		store.marked[uid] = struct{}{}
 	}
+	missing := make([]uint32, 0)
+	for _, uid := range visUids {
+		if msg, _ := store.Messages[uid]; msg == nil {
+			missing = append(missing, uid)
+		}
+	}
+	store.FetchHeaders(missing, nil)
 }
 
 func (store *MessageStore) NextPrev(delta int) {