slcanpty: detect that master PTY descriptor closed

Terminate slcanpty when the pty file descriptor is closed instead of performing
an endless loop in the select() statement.

Reported-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>

slcanpty.c

@@ -75,8 +75,11 @@ int pty2can(int pty, int socket, struct can_filter *fi,
 	int tmp, i;
 
 	nbytes = read(pty, &buf, sizeof(buf)-1);
-	if (nbytes < 0) {
+	if (nbytes <= 0) {
+		/* nbytes == 0 : no error but pty decriptor has been closed */
+		if (nbytes < 0)
 			perror("read pty");
+
 		return 1;
 	}