DataFlash: Fix logging error for EKF second instance

libraries/DataFlash/LogFile.cpp

@@ -1492,6 +1492,7 @@ void DataFlash_Class::Log_Write_EKF2(AP_AHRS_NavEKF &ahrs, bool optFlowEnabled)
 
         // Write 9th EKF packet
         ahrs.get_NavEKF2().getVariances(1,velVar, posVar, hgtVar, magVar, tasVar, offset);
+        magLength = magVar.length();
         ahrs.get_NavEKF2().getFilterFaults(1,faultStatus);
         ahrs.get_NavEKF2().getFilterTimeouts(1,timeoutStatus);
         ahrs.get_NavEKF2().getFilterStatus(1,solutionStatus);