demux: libavi: fix heap write ofw
demux: libavi: fix heap write ofw

(cherry picked from commit dde5f4aff32b81b6cace0a88a3a718147d0f3bf4)
Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2