Fix mixer issue with undefined return in callback and non value initialized variables.

src/systemcmds/tests/test_mixer.cpp

@@ -590,6 +590,7 @@ bool MixerTest::mixerTest()
 static int
 mixer_callback(uintptr_t handle, uint8_t control_group, uint8_t control_index, float &control)
 {
+	control = 0.0f;
 	if (control_group != 0) {
 		return -1;
 	}