AP_NavEKF3: set terrainState to zero on height datum reset

libraries/AP_NavEKF3/AP_NavEKF3_PosVelFusion.cpp

@@ -227,8 +227,10 @@ bool NavEKF3_core::resetHeightDatum(void)
         }
     }
 
-    // adjust the terrain state
-    terrainState += oldHgt;
+    // set the terrain state to zero (on ground). The adjustment for
+    // frame height will get added in the later constraints
+    terrainState = 0;
+
     return true;
 }