CONTRIB-6873 teacher view: adjust paging

Prevent the paging offset from being beyond the list's end

teacherview.php

@@ -438,6 +438,9 @@ function scheduler_print_schedulebox(scheduler_instance $scheduler, $studentid,
         $offset = 0;
     }
 }
+if ($offset * $pagesize >= $sqlcount && $sqlcount > 0) {
+    $offset = floor(($sqlcount-1) / $pagesize);
+}
 
 $slots = $scheduler->get_slots_for_teacher($teacherid, $currentgroup, $offset * $pagesize, $pagesize);