avformat/asfdec: Fix DoS in asf_build_simple_index()

Fixes: Missing EOF check in loop
No testcase

Found-by: Xiaohei and Wangchu from Alibaba Security Team
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static int asf_read_close(AVFormatContext *s)
{
    asf_reset_header(s);

    return 0;
}

static int asf_read_close(AVFormatContext *s)
{
    asf_reset_header(s);

    return 0;
}
