Fix redstone comparator flooding crash

Redstone comparators have two modes, comparison mode & subtraction mode.
Before this patch, the functions to turn comparators on or off attempted
to swap nodes with comparators in the same mode, but failed to determine
the correct replacement node, if the existing node was not a comparator.

When a comparator in an on state (e.g. powered by a filled cauldron) was
flooded, the flooding dropped the comparator and replaced the comparator
node that was to be swapped out with air, which lead to a server crash.

This patch changes the functions that turn comparators on or off so they
only swap existing nodes with comparators in the same mode if the name
of the replacement node can be determined – i.e. if it is not nil.

mods/ITEMS/REDSTONE/mcl_comparators/init.lua

@@ -43,14 +43,20 @@ end
 
 local function comparator_activate(pos, node)
 	local def = minetest.registered_nodes[node.name]
-	minetest.swap_node(pos, { name = def.comparator_onstate, param2 = node.param2 })
+	local onstate = def.comparator_onstate
+	if onstate then
+		minetest.swap_node(pos, { name = onstate, param2 = node.param2 })
+	end
 	minetest.after(0.1, comparator_turnon , {pos = pos, node = node})
 end
 
 
 local function comparator_deactivate(pos, node)
 	local def = minetest.registered_nodes[node.name]
-	minetest.swap_node(pos, { name = def.comparator_offstate, param2 = node.param2 })
+	local offstate = def.comparator_offstate
+	if offstate then
+		minetest.swap_node(pos, { name = offstate, param2 = node.param2 })
+	end
 	minetest.after(0.1, comparator_turnoff, {pos = pos, node = node})
 end