Fix freeze in core.check_for_falling

[savilli]

core.add_entity(pos, "__builtin:falling_node") can fail if there are too many entities already. In this case, convert_to_falling_node

minetest/builtin/game/falling.lua

Lines 412 to 414 in 7b3ed32

	local obj = core.add_entity(pos, "__builtin:falling_node")
	if not obj then
	return false

returns false, and the falling node is not removed. But core.check_single_for_falling didn't forward the error

minetest/builtin/game/falling.lua

Lines 532 to 533 in 7b3ed32

	convert_to_falling_node(p, n)
	return true

. Therefore core.check_for_falling tries to remove this node again

minetest/builtin/game/falling.lua

Lines 617 to 620 in 7b3ed32

	-- If we did need to walk the neighbor, then
	-- start walking it from the walk order start (1),
	-- and not the order we just pushed up the stack.
	v = 1

and the server freezes in an infinite loop.

To do

This PR is Ready for Review.

How to test

• Replace local obj = core.add_entity(pos, "__builtin:falling_node") with local obj = nil
• Place a falling node, for example, a sand
• Check that the server is not frozen

[sfan5]

lgtm

[SmallJoker]

Tested briefly; seems to work.