url_handlers: Cast the NULL pointer where required
url_handlers: Cast the NULL pointer where required

For variadic functions that take a NULL pointer as their last argument,
you should cast the NULL pointer to (char *)

In this case we should do that for TMPL_add_var()

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@opentechlabs.co.uk>

ac000/receiptomatic