test_ppm: add argc check

src/drivers/test_ppm/test_ppm.cpp

@@ -249,23 +249,26 @@ usage()
 int
 test_ppm_main(int argc, char *argv[])
 {
+	if (argc < 2) {
+		test_ppm::usage();
+		return -1;
+	}
+
 	const char *verb = argv[1];
 	unsigned  channels = 7;
 
 	/*
 	 * Start/load the driver.
-
 	 */
-
 	if (!strcmp(verb, "start")) {
 		test_ppm::start(channels);
-		exit(0);
+		return 0;
 	}
 
 	if (!strcmp(verb, "stop")) {
 
 		test_ppm::stop();
-		exit(0);
+		return 0;
 	}
 
 	/*
@@ -280,11 +283,9 @@ test_ppm_main(int argc, char *argv[])
 		unsigned value	= strtol(argv[3], NULL, 0);
 
 		test_ppm::set(channel, value);
-		exit(0);
+		return 0;
 	}
 
-
-
 	test_ppm::usage();
-	exit(1);
+	return -1;
 }