BT_MOVE(): call bt_put(_dst)

BT_MOVE(): call bt_put(_dst)

It makes no semantical sense to move a reference from a variable
to another without first putting the destination, if it has one.
Otherwise this would be a lost reference, unless it was moved
elsewhere or put manually using bt_put() previously, which is why
BT_MOVE() should always be used for move semantics and BT_PUT() is
preferred to bt_put() to avoid this situation.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>