ffserver: Check chunk size

Fixes out of array access

Fixes: poc_ffserver.py
Found-by: Paul Cher <paulcher@icloud.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
static HTTPContext *find_rtp_session(const char *session_id)
{
    HTTPContext *c;

    if (session_id[0] == '\0')
        return NULL;

    for(c = first_http_ctx; c; c = c->next) {
        if (!strcmp(c->session_id, session_id))
            return c;
    }
    return NULL;
}

static HTTPContext *find_rtp_session(const char *session_id)
{
    HTTPContext *c;

    if (session_id[0] == '\0')
        return NULL;

    for(c = first_http_ctx; c; c = c->next) {
        if (!strcmp(c->session_id, session_id))
            return c;
    }
    return NULL;
}
