Make shortpath/indent code guards explicit.

In actuality, the parent_depth and indent_width won't ever be
negative.

But the new code for unset $sidebar_relative_shortpath_indent makes
this much less obvious. Better to be explicit than have some small
future code change result in a bug.

sidebar.c

···
         }
       }
 
-      if (option (OPTSIDEBARSHORTPATH) && parent_depth)
+      if (option (OPTSIDEBARSHORTPATH) && (parent_depth > 0))
       {
         for (i = 0; parent_depth && sidebar_folder_name[i]; i++)
           if (strchr (SidebarDelimChars, sidebar_folder_name[i]))
···
         sidebar_folder_name += i;
       }
 
-      if (option (OPTSIDEBARFOLDERINDENT) && indent_width)
+      if (option (OPTSIDEBARFOLDERINDENT) && (indent_width > 0))
       {
         mutt_buffer_clear (indent_folder_name);
         for (i = 0; i < indent_width; i++)