OpticalFlow: init _last_update_ms member

This resolves a defect found with coverity

libraries/AP_OpticalFlow/OpticalFlow.cpp

@@ -43,12 +43,13 @@ const AP_Param::GroupInfo OpticalFlow::var_info[] PROGMEM = {
 // default constructor
 OpticalFlow::OpticalFlow(void) :
 #if CONFIG_HAL_BOARD == HAL_BOARD_PX4
-    backend(new AP_OpticalFlow_PX4(*this))
+    backend(new AP_OpticalFlow_PX4(*this)),
 #elif CONFIG_HAL_BOARD == HAL_BOARD_SITL
-    backend(new AP_OpticalFlow_HIL(*this))
+    backend(new AP_OpticalFlow_HIL(*this)),
 #else
-    backend(NULL)
+    backend(NULL),
 #endif
+    _last_update_ms(0)
 {
     AP_Param::setup_object_defaults(this, var_info);