AP_NavEKF3: review fixes

thanks Paul!

libraries/AP_NavEKF3/AP_NavEKF3.cpp

@@ -712,6 +712,9 @@ bool NavEKF3::InitialiseFilter(void)
     // Set the primary initially to be the lowest index
     primary = 0;
 
+    // invalidate shared origin
+    common_origin_valid = false;
+
     // initialise the cores. We return success only if all cores
     // initialise successfully
     bool ret = true;

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -481,7 +481,6 @@ bool NavEKF3_core::setOriginLLH(const Location &loc)
 // Set the NED origin to be used until the next filter reset
 void NavEKF3_core::setOrigin(const Location &loc)
 {
-    // assume origin at current GPS location (no averaging)
     EKF_origin = loc;
     // if flying, correct for height change from takeoff so that the origin is at field elevation
     if (inFlight) {