bug #36905 [Validator] Catch expected ValueError (derrabus)

This PR was merged into the 3.4 branch.

Discussion
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[Validator] Catch expected ValueError

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | #36872
| License       | MIT
| Doc PR        | N/A

`mb_check_encoding()` raises a `ValueError` on php 8 if an invalid encoding was passed. The validator that was patched here is expected to fail gracefully in that situation. This PR restores that behavior under php 8.

Maybe we can reconsider this behavior for Symfony 5.2. It feels a bit odd to me because we swallow a potential misconfiguration of the validator.

Commits
-------

8f3f67f [Validator] Catch expected ValueError.

src/Symfony/Component/Validator/Constraints/LengthValidator.php

@@ -39,8 +39,14 @@ public function validate($value, Constraint $constraint)
 
         $stringValue = (string) $value;
 
-        if (!$invalidCharset = !@mb_check_encoding($stringValue, $constraint->charset)) {
-            $length = mb_strlen($stringValue, $constraint->charset);
+        try {
+            $invalidCharset = !@mb_check_encoding($stringValue, $constraint->charset);
+        } catch (\ValueError $e) {
+            if (!str_starts_with($e->getMessage(), 'mb_check_encoding(): Argument #2 ($encoding) must be a valid encoding')) {
+                throw $e;
+            }
+
+            $invalidCharset = true;
         }
 
         if ($invalidCharset) {
@@ -54,6 +60,8 @@ public function validate($value, Constraint $constraint)
             return;
         }
 
+        $length = mb_strlen($stringValue, $constraint->charset);
+
         if (null !== $constraint->max && $length > $constraint->max) {
             $this->context->buildViolation($constraint->min == $constraint->max ? $constraint->exactMessage : $constraint->maxMessage)
                 ->setParameter('{{ value }}', $this->formatValue($stringValue))