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 int64_t get_server_clock(HTTPContext *c)
{
    /* compute current pts value from system time */
    return (cur_time - c->start_time) * 1000;
}

static int64_t get_server_clock(HTTPContext *c)
{
    /* compute current pts value from system time */
    return (cur_time - c->start_time) * 1000;
}
