demux: fix a recurrent warning
demux: fix a recurrent warning

by default DEMUX_TEST_AND_CLEAR_FLAGS returns EGENERIC and falls back in legacy
code

Signed-off-by: Thomas Guillem <5f50a84c1fa3bcff146405017f36aec1a10a9e38@gllm.fr>

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc