MDL-39249 Fix typo in available_update_deployer class

I have accidentally spotted this typo while working on this branch. It
has never caused any harm as both parameters callerurl and returnurl are
always passed. Blame copy/pasting of code blocks.

lib/pluginlib.php

@@ -2211,7 +2211,7 @@ class available_update_deployer {
         if (!empty($this->callerurl)) {
             $data['callerurl'] = $this->callerurl->out(false);
         }
-        if (!empty($this->callerurl)) {
+        if (!empty($this->returnurl)) {
             $data['returnurl'] = $this->returnurl->out(false);
         }