lavf/mux: pass options to nested structs of priv data

This is continuation of commit 330d547e
Nested struct is set in two places.
Previous commit set nested struct only in one case.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>