Fix for issue #55 not being able to enable online registration

This stops an empty api request from being made and causing an error.

CRM/Civirules/Utils/CustomDataFromPre.php

@@ -44,7 +44,7 @@ class CRM_Civirules_Utils_CustomDataFromPre {
      *   [] => value_b
      *
      */
-    if (CRM_Civirules_Utils_CustomField::isCustomFieldMultiselect($field_id) && is_array($value)) {
+    if ($field_id > 0 && CRM_Civirules_Utils_CustomField::isCustomFieldMultiselect($field_id) && is_array($value)) {
       $all_ones = true;
       foreach($value as $i => $j) {
         if ($j != 1) {
@@ -72,4 +72,4 @@ class CRM_Civirules_Utils_CustomDataFromPre {
 
 
 
-}
\ No newline at end of file
+}