Canvas: Set pointers to NULL instead of calling .release()

The Canvas camera and texture get recreated in several circumstances.
Calling .release() on a ref_ptr will just return the raw pointer and
'delete' won't be called. The proper way to explicitly delete an object
managed by a ref_ptr is to set the ref_ptr to NULL. This will delete it
as long as it's not referenced by any other ref_ptr.

simgear/canvas/ODGauge.cxx

@@ -298,8 +298,8 @@ namespace canvas
   {
     if( camera.valid() && Canvas::getSystemAdapter() )
       Canvas::getSystemAdapter()->removeCamera(camera.get());
-    camera.release();
-    texture.release();
+    camera = nullptr;
+    texture = nullptr;
 
     _flags &= ~AVAILABLE;
   }