Clean up door tops if fire destroys bottom.

Fixes #909. Door tops are never flammable.

This doesn't guard yet against a voxelmanip removing the top node,
but that is less of an issue since if a voxelmanip removes the top,
then the bottom part remains functional and visibly intact. If the
voxelmanip removes the bottom part, but not the top, then this patch
makes it clean up the top just fine.

mods/doors/init.lua

@@ -339,6 +339,10 @@ function doors.register(name, def)
 		end
 	end
 
+	def.on_destruct = function(pos)
+		minetest.remove_node({x = pos.x, y = pos.y + 1, z = pos.z})
+	end
+
 	minetest.register_node(":" .. name .. "_a", {
 		description = def.description,
 		visual = "mesh",
@@ -361,6 +365,7 @@ function doors.register(name, def)
 		can_dig = def.can_dig,
 		on_rotate = def.on_rotate,
 		on_blast = def.on_blast,
+		on_destruct = def.on_destruct,
 		selection_box = {
 			type = "fixed",
 			fixed = { -1/2,-1/2,-1/2,1/2,3/2,-6/16}
@@ -393,6 +398,7 @@ function doors.register(name, def)
 		can_dig = def.can_dig,
 		on_rotate = def.on_rotate,
 		on_blast = def.on_blast,
+		on_destruct = def.on_destruct,
 		selection_box = {
 			type = "fixed",
 			fixed = { -1/2,-1/2,-1/2,1/2,3/2,-6/16}