MDL-26892 questions: images lost during upgrade.

This affects questions shared in the system or course category areas of
the question bank.

Thanks to Chris Edwards for working out what was going wrong.

lib/db/upgrade.php

@@ -5034,7 +5034,8 @@ function xmldb_main_upgrade($oldversion) {
                         break;
                     case CONTEXT_COURSECAT :
                     case CONTEXT_SYSTEM :
-                        $context = get_system_context();
+                        // Stored in the front-page course.
+                        $context = get_context_instance(CONTEXT_COURSE, get_site()->id);
                         break;
                     default :
                         continue;