Don't fail when it wasn't possible to fetch the interface name.

On some systems SIOCGIFNAME may fail with ENOTTY, but the actual
slcanX interface gets properly configured. Instead of crashing hard
on such case, let's gracefuly degrade and just not display the
interface name.

slcand.c

@@ -370,8 +370,13 @@ int main(int argc, char *argv[])
 
 	/* retrieve the name of the created CAN netdevice */
 	if (ioctl(fd, SIOCGIFNAME, buf) < 0) {
-		perror("ioctl SIOCGIFNAME");
-		exit(EXIT_FAILURE);
+		if (name) {
+			perror("ioctl SIOCGIFNAME");
+			exit(EXIT_FAILURE);
+		} else {
+			/* Graceful degradation: we only needed the name for display. */
+			snprintf(buf, sizeof(buf), "<unknown>");
+		}
 	}
 
 	syslogger(LOG_NOTICE, "attached TTY %s to netdevice %s\n", ttypath, buf);