macro_start_args truncates def_buf, but does not check to ensure that
someone has allocated def_buf first. This will cause a null pointer
dereference if the first access to def_buf while parsing a file is a
macro invocation. Fix this by avoiding truncating def_buf if it is NULL,
as it is effectively already truncated.
Fixes: 680196953 ("Add support for text macros with arguments.")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Sean Anderson