mixer test: Fix string handling

Some strings were not enforcing NUL termination.

src/systemcmds/tests/test_mixer.cpp

@@ -216,7 +216,9 @@ bool MixerTest::loadAllTest()
 			if (strncmp(result->d_name, ".", 1) != 0) {
 
 				char buf[PATH_MAX];
-				(void)strncpy(&buf[0], MIXER_ONBOARD_PATH, sizeof(buf));
+				(void)strncpy(&buf[0], MIXER_ONBOARD_PATH, sizeof(buf) - 1);
+				/* enforce null termination */
+				buf[sizeof(buf) - 1] = '\0';
 				(void)strncpy(&buf[strlen(MIXER_ONBOARD_PATH)], "/", 1);
 				(void)strncpy(&buf[strlen(MIXER_ONBOARD_PATH) + 1], result->d_name, sizeof(buf) - strlen(MIXER_ONBOARD_PATH) - 1);
 
@@ -597,7 +599,7 @@ mixer_callback(uintptr_t handle, uint8_t control_group, uint8_t control_index, f
 		return -1;
 	}
 
-	if (control_index > (sizeof(actuator_controls) / sizeof(actuator_controls[0]))) {
+	if (control_index >= (sizeof(actuator_controls) / sizeof(actuator_controls[0]))) {
 		return -1;
 	}