avfilter: avoid zero arguments to variadic macro

avfilter: avoid zero arguments to variadic macro

ISO C requires at least one argument in the place of the ellipsis in a
variadic macro. In particular, under -pedantic, this triggers the
warning -Wgnu-zero-variadic-macro-arguments on clang 3.7.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>