PHP74兼容性问题

修复一处导致PHP74无法正常执行的符号

app/function/common.function.php

@@ -1028,7 +1028,7 @@ function des_encode($key, $text){
 	return base64_encode($encrypted);
 } 
 function pkcs5_unpad($text){
-	$pad = ord($text{strlen($text)-1});
+	$pad = ord($text[strlen($text)-1]);
 	if ($pad > strlen($text)) return $text;
 	if (strspn($text, chr($pad), strlen($text) - $pad) != $pad) return $text;
 	return substr($text, 0, -1 * $pad);
@@ -1036,4 +1036,4 @@ function pkcs5_unpad($text){
 function pkcs5_pad($text, $block = 8){
 	$pad = $block - (strlen($text) % $block);
 	return $text . str_repeat(chr($pad), $pad);
-}
+}