Reset snap point variables in DragManager.drag_end

In the `drag_end` function, make sure to reset `previewing_snap_point`
in addition to `preview_block`. This ensures that block snapping will
work correctly if the same block is dragged a second time. In addition,
defer cleaning up preview_block and previewing_snap_point to the end of
the function. This avoids some confusion related to nested if
statements.

https://phabricator.endlessm.com/T35524

Author: dylanmccall

addons/block_code/drag_manager/drag_manager.gd

@@ -16,7 +16,6 @@ var dragging: Block = null
 
 var previewing_snap_point: SnapPoint = null
 var preview_block: Control = null
-var preview_owner: Block = null
 
 var _picker: Picker
 var _block_canvas: BlockCanvas
@@ -132,16 +131,15 @@ func drag_ended():
 
 		# Check if in BlockCanvas
 		var block_canvas_rect: Rect2 = _block_canvas.get_global_rect()
+
 		if block_canvas_rect.encloses(block_rect):
 			dragging.disconnect_signals()  # disconnect previous on canvas signal connections
 			connect_block_canvas_signals(dragging)
 			remove_child(dragging)
 			dragging.on_canvas = true
 
-			if preview_block:
+			if previewing_snap_point:
 				# Can snap block
-				preview_block.free()
-				preview_block = null
 				var orphaned_block = previewing_snap_point.set_snapped_block(dragging)
 				if orphaned_block:
 					# Place the orphan block somewhere outside the snap point
@@ -156,7 +154,13 @@ func drag_ended():
 		else:
 			dragging.queue_free()
 
+		if preview_block:
+			preview_block.queue_free()
+			preview_block = null
+
+		previewing_snap_point = null
 		dragging = null
+
 		block_dropped.emit()