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 void rtsp_reply_error(HTTPContext *c, enum RTSPStatusCode error_number)
{
    rtsp_reply_header(c, error_number);
    avio_printf(c->pb, "\r\n");
}

static void rtsp_reply_error(HTTPContext *c, enum RTSPStatusCode error_number)
{
    rtsp_reply_header(c, error_number);
    avio_printf(c->pb, "\r\n");
}
