Real pseudo-RTSP: prevent integer underflow

Pointed-out-by: tixxDZ <tixxdz@gmail.com> - DZCORE Labs, Algeria
(cherry picked from commit dc74600c)

modules/access/rtsp/real.c

@@ -592,6 +592,7 @@ int real_get_rdt_chunk(rtsp_client_t *rtsp_session, rmff_pheader_t *ph,
 
   int n;
   rmff_dump_pheader(ph, (char*)*buffer);
+  if (pf->length<12) return 0;
   n=rtsp_read_data(rtsp_session, (uint8_t*)(*buffer + 12), ph->length - 12);
   return (n <= 0) ? 0 : n+12;
 }