AVFMTCTX_NOHEADER fix

I noticed the on some formats, VOB file namely, new streams can appear after a av_read_frame and AVFormatContext.nb_streams will increase. This stops it from raising a exception at least. The docs say AVFMTCTX_NOHEADER is a indicator that this might happen. http://ffmpeg.org/doxygen/trunk/structAVFormatContext.html#a0b748d924898b08b89ff4974afd17285