Added check to avoid notices when trying to get grades from an assignment

that didn't have any grades yet.    bug 4287

Also, using datalib function to create LIMIT statement when displaying
submissions

mod/assignment/lib.php

@@ -660,7 +660,7 @@ class assignment_base {
         } else {
             $currentgroup = false;
         }
-        $limit = " LIMIT ".($offset+1).", 1";
+        $limit = sql_paging_limit($offset+1, 1);
 
     /// Get all teachers and students
         if ($currentgroup) {
@@ -1558,9 +1558,11 @@ function assignment_grades($assignmentid) {
                                $assignment->id, '', 'userid,grade');
 
     if ($assignment->grade > 0) {
-        foreach ($grades as $userid => $grade) {
-            if ($grade == -1) {
-                $grades[$userid] = '-';
+        if ($grades) {
+            foreach ($grades as $userid => $grade) {
+                if ($grade == -1) {
+                    $grades[$userid] = '-';
+                }
             }
         }
         $return->grades = $grades;