normal/completion: Fix possible NULL pointer dereference

Coverity Scan reports that the grub_strrchr() function can return NULL if
the character is not found. Check if that's the case for dirfile pointer.

Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/normal/completion.c

@@ -284,7 +284,8 @@ complete_file (void)
 
       /* Cut away the filename part.  */
       dirfile = grub_strrchr (dir, '/');
-      dirfile[1] = '\0';
+      if (dirfile)
+	dirfile[1] = '\0';
 
       /* Iterate the directory.  */
       (fs->fs_dir) (dev, dir, iterate_dir, NULL);