Fix NULL pointer dereference in read_post_data()

What took down cvs/sbbs yesterday:
Program terminated with signal SIGSEGV, Segmentation fault.
6203                    session->req.post_data[session->req.post_len]=0;
[Current thread is 1 (Thread 0x7f2b989ff700 (LWP 17031))]
(gdb) print post_len
No symbol "post_len" in current context.
(gdb) print session->req.post_len
$1 = 0
(gdb) print session->req.post_data
$2 = 0x0

src/sbbs3/websrvr.c

@@ -6200,7 +6200,8 @@ int read_post_data(http_session_t * session)
 				lprintf(LOG_DEBUG,"%04d !ERROR Browser said they sent %lu bytes, but I got %lu",session->socket, (ulong)s, (ulong)session->req.post_len);
 		if(session->req.post_len > s)
 			session->req.post_len = s;
-		session->req.post_data[session->req.post_len]=0;
+		if(session->req.post_data != NULL)
+			session->req.post_data[session->req.post_len]=0;
 	}
 	return(TRUE);
 }