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;
 	}