ffmpeg: check timebase validity before considering it to be used for stream copy

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -2055,7 +2055,8 @@ static int transcode_init(void)
                       && strcmp(oc->oformat->name, "mov") && strcmp(oc->oformat->name, "mp4") && strcmp(oc->oformat->name, "3gp")
                       && strcmp(oc->oformat->name, "3g2") && strcmp(oc->oformat->name, "psp") && strcmp(oc->oformat->name, "ipod")
             ) {
-                if(   copy_tb<0 && av_q2d(icodec->time_base)*icodec->ticks_per_frame > av_q2d(ist->st->time_base)
+                if(   copy_tb<0 && icodec->time_base.den
+                                && av_q2d(icodec->time_base)*icodec->ticks_per_frame > av_q2d(ist->st->time_base)
                                 && av_q2d(ist->st->time_base) < 1.0/500
                    || copy_tb==0){
                     codec->time_base = icodec->time_base;