More strict check of the length argument for a network read.

lib/Predis.php

@@ -1380,7 +1380,7 @@ class Connection implements IConnection {
     }
 
     public function readBytes($length) {
-        if ($length == 0) {
+        if ($length <= 0) {
             throw new \InvalidArgumentException('Length parameter must be greater than 0');
         }
         $socket = $this->getSocket();