lib: move msg validity check into separate function
lib: move msg validity check into separate function

Signed-off-by: Jiri Pirko <1b548d10035ae428195c41f5d55eedb1437ccd8a@resnulli.us>

jpirko/libndp,AGWA/libndp-debian,sgros/MIF_libndp,AGWA/libndp-debian,sgros/MIF_libndp,jpirko/libndp