AP_NavEKF3: correct compilation when running MATH_CHECK_INDEXES

libraries/AP_NavEKF3/AP_NavEKF3_core.cpp

@@ -484,7 +484,7 @@ bool NavEKF3_core::InitialiseFilterBootstrap(void)
 void NavEKF3_core::CovarianceInit()
 {
     // zero the matrix
-    memset(P, 0, sizeof(P));
+    memset(&P[0][0], 0, sizeof(P));
 
     // define the initial angle uncertainty as variances for a rotation vector
     Vector3f rot_vec_var;