Fix text field resetting text when replaced with a short string

Triggered by auto-filling a password with less than 16 characters from a
password manager.

Author: frenzibyte

src/video/uikit/SDL_uikitviewcontroller.m

@@ -633,7 +633,7 @@ - (void)setKeyboardHeight:(int)height
 - (BOOL)textField:(UITextField *)_textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
 {
     if (textField.markedTextRange == nil) {
-        if (textField.text.length < 16) {
+        if ([string length] == 0 && textField.text.length < 16) {
             [self resetTextState];
         }
     }