Fix issues related to #419.

src/Environment/terrainsampler.cxx

@@ -371,6 +371,7 @@ void TerrainSamplerImplementation::reinit()
         if( subsys == NULL )
             break;
         remove_subsystem( subsystemName );
+        subsys->unbind();
         delete subsys;
     }
     

src/Model/acmodel.cxx

@@ -55,6 +55,8 @@ FGAircraftModel::FGAircraftModel ()
 
 FGAircraftModel::~FGAircraftModel ()
 {
+  // drop reference
+  _fx = 0;
   deinit();
 }
 
@@ -91,9 +93,6 @@ FGAircraftModel::reinit()
 void
 FGAircraftModel::deinit()
 {
-  // drop reference
-  _fx = 0;
-
   if (!_aircraft) {
     return;
   }