fix off-by-one in argument handling

Signed-off-by: Steve Markgraf <steve@steve-m.de>

src/main.c

@@ -141,7 +141,7 @@ int main(int argc, char **argv)
 		filename = argv[optind];
 	}
 #else
-	if(argc <5)
+	if(argc <6)
 		usage();
 	dev_index = atoi(argv[1]);
 	samp_rate = atoi(argv[2])*1000;