Fix crash when no message is selected

Pressing `Enter` on a view that has not yet loaded messages (e.g. at
startup) would return `nil` from `Selected()`. Accessing `msg.Uid` on a
`nil` reference crashes aerc.

This patch moves the `msg == nil` check before accessing `msg.Uid` thus
avoiding the crash.

To test this patch repeatedly press `Enter` on startup.

commands/account/view.go

@@ -30,8 +30,11 @@ func (ViewMessage) Execute(aerc *widgets.Aerc, args []string) error {
 	}
 	store := acct.Messages().Store()
 	msg := acct.Messages().Selected()
+	if msg == nil {
+		return nil
+	}
 	_, deleted := store.Deleted[msg.Uid]
-	if msg == nil || deleted {
+	if deleted {
 		return nil
 	}
 	viewer := widgets.NewMessageViewer(acct, aerc.Config(), store, msg)