AP_NavEKF: Prevent divide by zero casued by Tnb_flow.c.z = 0

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -926,14 +926,14 @@ void NavEKF::SelectFlowFusion()
         // update the time stamp
         prevFlowFusionTime_ms = imuSampleTime_ms;
 
-    } else if (flowDataValid && flow_state.obsIndex == 1 && !delayFusion && !constPosMode){
+    } else if (flowDataValid && flow_state.obsIndex == 1 && !delayFusion && !constPosMode && tiltOK){
         // Fuse the optical flow Y axis data into the main filter
         FuseOptFlow();
         // increment the index to fuse the X and Y data using the 2-state EKF on the next prediction cycle
         flow_state.obsIndex = 2;
         // indicate that flow fusion has been performed. This is used for load spreading.
         flowFusePerformed = true;
-    } else if (((flowDataValid && flow_state.obsIndex == 2) || newDataRng) && !constPosMode) {
+    } else if (((flowDataValid && flow_state.obsIndex == 2) || newDataRng) && !constPosMode && tiltOK) {
         // enable fusion of range data if available and permitted
         if(newDataRng && useRngFinder()) {
             fuseRngData = true;