demux: mp4: fix divbyzero with broken audio stsd
demux: mp4: fix divbyzero with broken audio stsd

(cherry picked from commit 9e2ed89062e3d47b66fa7f9611fce20d5deda2a6)
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