MJPEG: Jump on uninitialized variable

In the MJPEG demux:

    b_matched = CheckMimeHeader( p_demux, &i_size);
    if( b_matched )
    {
        p_demux->pf_demux = MimeDemux;
        stream_Read( p_demux->s, NULL, i_size );
    }
    else if( 0 == i_size )

i_size is not always initialized when CheckMimeHeader() returns false.