Fix two kelp-related crashes, add kelp detach logging

mods/ITEMS/mcl_ocean/kelp.lua

@@ -185,6 +185,7 @@ end
 
 
 -- Obtain position of the first kelp unsubmerged.
+-- Returns: pos, node, height, break_height
 function kelp.find_unsubmerged(pos, node, height)
 	-- Optional params: node, height
 	local node = node or mt_get_node(pos)
@@ -332,10 +333,15 @@ function kelp.detach_dig(dig_pos, pos, drop, node, height)
 		if drop then
 			kelp.detach_drop(dig_pos, height)
 		end
-		mt_set_node(pos, {
-			name=mt_registered_nodes[node.name].node_dig_prediction,
-			param=node.param,
-			param2=0 })
+
+		-- Only set the node if the node would be changed
+		local dig_prediction = mt_registered_nodes[node.name].node_dig_prediction
+		if dig_prediction and dig_prediction ~= node.name then
+			mt_set_node(pos, {
+				name=dig_prediction,
+				param=node.param,
+				param2=0 })
+		end
 
 	-- Digs the kelp beginning at a height.
 	else
@@ -363,9 +369,15 @@ end
 local function detach_unsubmerged(pos)
 	local node = mt_get_node(pos)
 
-	local dig_pos,_, height = kelp.find_unsubmerged(pos, node)
+	local dig_pos,node, height, break_height = kelp.find_unsubmerged(pos, node)
 	if dig_pos then
-		mt_sound_play(mt_registered_nodes[node.name].sounds.dug, { gain = 0.5, pos = dig_pos }, true)
+		if node then
+			minetest.log("info","Detaching kelp because of "..node.name.." at "..vector.to_string(vector.offset(dig_pos,0,break_height,0)))
+		end
+		local node_def = mt_registered_nodes[node.name]
+		if node_def and node_def.sounds then
+			mt_sound_play(node_def.sounds.dug, { gain = 0.5, pos = dig_pos }, true)
+		end
 		kelp.detach_dig(dig_pos, pos, true, node, height)
 		local new_age = kelp.roll_init_age()
 		store_age(pos, new_age)