MDL-37838 block_courseoverview: fix pluaralisation logic

Note that this kind of handling of plurals is extremely limited, see:

https://moodle.org/mod/forum/discuss.php?d=85897
http://search.cpan.org/~petdance/Locale-Maketext-1.12/lib/Locale/Maketext/TPJ13.pod

However, this is a simple fix to the existing logic

blocks/course_overview/renderer.php

@@ -318,11 +318,11 @@ class block_course_overview_renderer extends plugin_renderer_base {
         $plural = 's';
         if ($msgcount > 0) {
             $output .= get_string('youhavemessages', 'block_course_overview', $msgcount);
-        } else {
-            $output .= get_string('youhavenomessages', 'block_course_overview');
             if ($msgcount == 1) {
                 $plural = '';
             }
+        } else {
+            $output .= get_string('youhavenomessages', 'block_course_overview');
         }
         $output .= html_writer::link(new moodle_url('/message/index.php'), get_string('message'.$plural, 'block_course_overview'));
         $output .= $this->output->box_end();