AP_NavEKF2: fixed build with g++ 9.1

this is a false positive, but the cost of clearing this array is low,
and it saves a much more complex fix

libraries/AP_NavEKF2/AP_NavEKF2_MagFusion.cpp

@@ -1040,7 +1040,7 @@ void NavEKF2_core::FuseDeclination(float declErr)
     float t12 = 1.0f/t11;
 
     float H_MAG[24];
-    Vector28 Kfusion;
+    Vector28 Kfusion {};
 
     H_MAG[16] = -magE*t5;
     H_MAG[17] = magN*t5;

libraries/AP_NavEKF2/AP_NavEKF2_OptFlowFusion.cpp

@@ -285,7 +285,7 @@ void NavEKF2_core::FuseOptFlow()
     Vector3f relVelSensor;
     Vector14 SH_LOS;
     Vector2 losPred;
-    Vector28 Kfusion;
+    Vector28 Kfusion {};
 
     // Copy required states to local variable names
     float q0  = stateStruct.quat[0];