demux: mp4: don't trust only timescale for rate
demux: mp4: don't trust only timescale for rate

wrong fix was 198bb199366ce6d0320fb5fca64784abec58c65f

krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,vlc-mirror/vlc,krichter722/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,shyamalschandra/vlc,xkfz007/vlc,krichter722/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,xkfz007/vlc,shyamalschandra/vlc,krichter722/vlc,krichter722/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc