MDL-60579 user: Check filter strings have correct part count

user/renderer.php

@@ -277,6 +277,9 @@ class core_user_renderer extends plugin_renderer_base {
         foreach ($filtersapplied as $filter) {
             if (!array_key_exists($filter, $filteroptions)) {
                 $filtervalue = explode(':', $filter);
+                if (count($filtervalue) !== 2) {
+                    continue;
+                }
                 $key = $filtervalue[0];
                 $value = $filtervalue[1];
 

user/tests/behat/filter_participants.feature

@@ -108,6 +108,7 @@ Feature: Course participants can be filtered
     Given I log in as "teacher1"
     And I am on "Course 1" course homepage
     And I navigate to course participants
+    # Note: This is the literal string "student", not the Role student.
     When I set the field "Filters" to "student"
     And I press key "13" in the field "Filters"
     Then I should see "Student 1" in the "participants" "table"