iOS 13 崩溃适配 2019-06-25

1. Exception Name: NSGenericException
   Exception Reason: Access to UITextField's _placeholderLabel ivar is prohibited. This is an application bug

 [self.textField setValue:[UIColor colorWithHexString:@"0x9598a2"] forKeyPath:@"_placeholderLabel.textColor"];
[self.textField setValue:font forKeyPath:@"_placeholderLabel.font"];