Fix placement of call to simplifyURL() function

Before this commit, the URL simplification was performed within the
$params array used as the *default* values, so these values got
over-written by the submitted values with array_merge().

This is a follow-up to changes made in civicrm#10720 bb80d2f

CRM/Contribute/BAO/ManagePremiums.php

@@ -98,13 +98,18 @@ public static function setIsActive($id, $is_active) {
   public static function add(&$params, &$ids) {
     $params = array_merge(array(
       'id' => CRM_Utils_Array::value('premium', $ids),
-      'image' => CRM_Utils_String::simplifyURL(CRM_Utils_Array::value('image', $params, ''), TRUE),
-      'thumbnail' => CRM_Utils_String::simplifyURL(CRM_Utils_Array::value('thumbnail', $params, ''), TRUE),
+      'image' => '',
+      'thumbnail' => '',
       'is_active' => 0,
       'is_deductible' => FALSE,
       'currency' => CRM_Core_Config::singleton()->defaultCurrency,
     ), $params);
 
+    // Modify the submitted values for 'image' and 'thumbnail' so that we use
+    // local URLs for these images when possible.
+    $params['image'] = CRM_Utils_String::simplifyURL($params['image'], TRUE);
+    $params['thumbnail'] = CRM_Utils_String::simplifyURL($params['thumbnail'], TRUE);
+
     // Save and return
     $premium = new CRM_Contribute_DAO_Product();
     $premium->copyValues($params);