WIP fix to issue #1053: strings being used to define PRT scores.

questiontype.php

@@ -1085,6 +1085,8 @@ class qtype_stack extends question_type {
 
         foreach ($q->prts as $index => $prt) {
             foreach ($prt->get_nodes_summary() as $nodeid => $choices) {
+                // STACK allows variables in scores, which may not be evaluated.
+                if (is_numeric($choices->falsescore) && is_numeric($choices->truescore)) {
                     $parts[$index . '-' . $nodeid] = array(
                         $choices->falseanswernote => new question_possible_response(
                                 $choices->falseanswernote, $choices->falsescore * $prt->get_value()),
@@ -1094,6 +1096,7 @@ class qtype_stack extends question_type {
                     );
                 }
             }
+        }
 
         return $parts;
     }