Fix buffer underflow when serial peer spews garbage

A board I got spews garbage on hard reset. Sometimes that garbage is IAC
and it leads to len here getting negative and the loop eventually
underflows the buf.

Detect and avoid this silently.

mux.c

@@ -306,7 +306,7 @@ static int handle_receive_buf(struct ios_ops *ios, unsigned char *buf, int len)
 	unsigned char *sendbuf = buf;
 	int i;
 
-	while (len) {
+	while (len > 0) {
 		switch (*buf) {
 		case IAC:
 			/* BUG: this is telnet specific */