fix the TextInput can't control input length when value's length > maxLength (#23545)

Summary:
I found the TextInput can't control input length when default value's length > maxLength.
for example：
1.Set the value in special cases
```
<TextInput value={'12345678'} maxLength={6}/>
```
2.Quickly press the keyboard with multiple fingers

```
// RCTBaseTextInputView.m
……
if (_maxLength) {
    NSUInteger allowedLength = _maxLength.integerValue - backedTextInputView.attributedText.string.length + range.length;
    if (text.length > allowedLength) {
……
```
when value's length > maxLength,the allowedLength not a negative number.it was transformed into a big number,because it is type NSUInteger.so the `text.length > allowedLength` always false.

[iOS][Fixed] - fix the TextInput can't control input length when value's length > maxLength
Pull Request resolved: #23545

Differential Revision: D14146581

Pulled By: cpojer

fbshipit-source-id: f53b1312ae55fad9fc10430ab94784c1a9ad4723

Author: jsfu

Libraries/Text/TextInput/RCTBaseTextInputView.m

@@ -305,9 +305,9 @@ - (BOOL)textInputShouldChangeTextInRange:(NSRange)range replacementText:(NSStrin
   }
 
   if (_maxLength) {
-    NSUInteger allowedLength = _maxLength.integerValue - backedTextInputView.attributedText.string.length + range.length;
+    NSInteger allowedLength = _maxLength.integerValue - backedTextInputView.attributedText.string.length + range.length;
 
-    if (text.length > allowedLength) {
+    if (allowedLength < 0 || text.length > allowedLength) {
       // If we typed/pasted more than one character, limit the text inputted.
       if (text.length > 1) {
         // Truncate the input string so the result is exactly maxLength