ios7 textViewDidChange代理/通知中修改TextView内容崩溃的原因

http://stackoverflow.com/questions/19948394/textviewdidchange-crashes-in-ios-7/19960476

http://wenda.baba.io/questions/601094/textviewdidchange-crashes-in-ios-7.html

当self.tv.markedTextRange为nil时再修改TextView内容：

if (self.tv.markedTextRange == nil && self.tv.text.length > maxLength) {

// Perform change

}