AP_NavEKF2: initialize stateStruct.quat to unit length

libraries/AP_NavEKF2/AP_NavEKF2_core.cpp

@@ -238,6 +238,7 @@ void NavEKF2_core::InitialiseVariables()
     runUpdates = false;
     framesSincePredict = 0;
     gpsYawResetRequest = false;
+    stateStruct.quat.initialise();
     quatAtLastMagReset = stateStruct.quat;
     delAngBiasLearned = false;
     memset(&filterStatus, 0, sizeof(filterStatus));