Use defaultrequestcategory for new course from print_courses() if no other category is available

print_courses() produced PHP notice (Trying to get property of
non-object) and generated an invalid button to create a new course in this
case: (1) Passed paramater $category is 0 to display all categores (as in
case of home page, for example) and (2) get_child_categories(0) returns
more than one record. In such case, $categories is still a plain
integer, does not contain ->id property and the called /course/edit.php
exits with error.
This patch uses $CFG->defaultrequestcategory as category id in this
case.

course/lib.php

@@ -2137,7 +2137,11 @@ function print_courses($category) {
         $context = get_context_instance(CONTEXT_SYSTEM);
         if (has_capability('moodle/course:create', $context)) {
             $options = array();
-            $options['category'] = $category->id;
+            if (!empty($category->id)) {
+                $options['category'] = $category->id;
+            } else {
+                $options['category'] = $CFG->defaultrequestcategory;
+            }
             echo '<div class="addcoursebutton">';
             echo $OUTPUT->single_button(new moodle_url('/course/edit.php', $options), get_string("addnewcourse"));
             echo '</div>';