AP_AHRS: protect against zero deltat in DCM

fixes issue #2657

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -119,10 +119,12 @@ AP_AHRS_DCM::matrix_update(float _G_Dt)
     if (healthy_count > 1) {
         delta_angle /= healthy_count;
     }
+    if (_G_Dt > 0) {
         _omega = delta_angle / _G_Dt;
         _omega += _omega_I;
         _dcm_matrix.rotate((_omega + _omega_P + _omega_yaw_P) * _G_Dt);
     }
+}
 
 
 /*
@@ -559,11 +561,13 @@ AP_AHRS_DCM::drift_correction(float deltat)
             float delta_velocity_dt;
             _ins.get_delta_velocity(i, delta_velocity);
             delta_velocity_dt = _ins.get_delta_velocity_dt(i);
+            if (delta_velocity_dt > 0) {
                 _accel_ef[i] = _dcm_matrix * (delta_velocity / delta_velocity_dt);
                 // integrate the accel vector in the earth frame between GPS readings
                 _ra_sum[i] += _accel_ef[i] * deltat;
             }
         }
+    }
 
     //update _accel_ef_blended
 #if HAL_CPU_CLASS >= HAL_CPU_CLASS_75