AP_NavEKF2: don't use WMM tables unless we have a compass scale factor set

libraries/AP_NavEKF2/AP_NavEKF2_Measurements.cpp

@@ -577,6 +577,8 @@ void NavEKF2_core::readGpsData()
             }
 
             if (gpsGoodToAlign && !have_table_earth_field) {
+                const Compass *compass = _ahrs->get_compass();
+                if (compass && compass->have_scale_factor(magSelectIndex)) {
                     table_earth_field_ga = AP_Declination::get_earth_field_ga(gpsloc);
                     table_declination = radians(AP_Declination::get_declination(gpsloc.lat*1.0e-7,
                                                                                 gpsloc.lng*1.0e-7));
@@ -586,6 +588,7 @@ void NavEKF2_core::readGpsData()
                         stateStruct.earth_magfield = table_earth_field_ga;
                     }
                 }
+            }
             
             // convert GPS measurements to local NED and save to buffer to be fused later if we have a valid origin
             if (validOrigin) {