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

src/modules/px4iofirmware/mixer.cpp

@@ -384,6 +384,8 @@ mixer_callback(uintptr_t handle,
 	       uint8_t control_index,
 	       float &control)
 {
+	control = 0.0f;
+
 	if (control_group >= PX4IO_CONTROL_GROUPS) {
 		return -1;
 	}

src/modules/systemlib/mixer/mixer_simple.cpp

@@ -299,7 +299,7 @@ SimpleMixer::mix(float *outputs, unsigned space)
 	}
 
 	for (unsigned i = 0; i < _pinfo->control_count; i++) {
-		float input;
+		float input = 0.0f;
 
 		_control_cb(_cb_handle,
 			    _pinfo->controls[i].control_group,

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;
 	}