Add missing return statement to _mb_ord()

[westonruter]

This is a follow-up to #11965.

There is a PHPStan rule level 0 error introduced in that PR:

 ------ ----------------------------------------------------------------------------- 
  Line   src/wp-includes/compat.php                                                   
 ------ ----------------------------------------------------------------------------- 
  227    Function _mb_ord() should return int|false but return statement is missing.  
         🪪  return.missing                                                           
         at src/wp-includes/compat.php:227                                            
 ------ ----------------------------------------------------------------------------- 

While it doesn't seem that $byte_length can ever be anything other than int<1, 4> when 1 !== $found_count, this is not picked up by PHPStan, leading ot the error.

An alternative to this would be to declare the type:

diff --git a/src/wp-includes/compat.php b/src/wp-includes/compat.php
index 5eb467280a..d67cea2d0c 100644
--- a/src/wp-includes/compat.php
+++ b/src/wp-includes/compat.php
@@ -221,6 +221,8 @@ function _mb_ord( $string, $encoding = null ) {
 		return false;
 	}
 
+	/** @var int<1, 4> $byte_length */
+
 	// These are valid code points, so no further validation is required.
 	$b0 = ord( $string[0] );

But this seems less clean.

Trac ticket: https://core.trac.wordpress.org/ticket/65342

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Props westonruter, dmsnell.

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[dmsnell]

Thanks @westonruter — the inline comment is probably better semantically but the return false would potentially make up for any regressions that would creep into _wp_scan_utf8(), as unlikely as those are.