Silence errors from iconv_strlen()

The return value of iconv_strlen() is checked, thus there is not need to have an error (E_NOTICE) when an illegal byte sequence is encountered in a file that is being checked.

CodeSniffer/File.php

@@ -1511,7 +1511,7 @@ private static function _createPositionMap(&$tokens, $tokenizer, $eolChar, $enco
                 // There are no tabs in this content, or we aren't replacing them.
                 if ($checkEncoding === true) {
                     // Not using the default encoding, so take a bit more care.
-                    $length = iconv_strlen($tokens[$i]['content'], $encoding);
+                    $length = @iconv_strlen($tokens[$i]['content'], $encoding);
                     if ($length === false) {
                         // String contained invalid characters, so revert to default.
                         $length = strlen($tokens[$i]['content']);