add nil checks and backguard compatiblity

* real fix for https://notabug.org/TenPlus1/mobs_redo/issues/112
  use right variable in mount.lua and detect player api
* adapt patch from https://notabug.org/TenPlus1/mobs_redo/issues/114
  about the rare rare crash of srt.find never got cause
* check right player privilegies only if player its valid for
  when player its really peaceful_player and not only mod flag
* check right player nature when check if mobs can follow
* active mobs its zero event negative amount
  fix for https://notabug.org/TenPlus1/mobs_redo/issues/144

api.lua

@@ -305,6 +305,8 @@ local function check_for(look_for, look_inside)
 		for _, str in pairs(look_inside) do
 
 			if not str then goto continue end
+			if not str or not str.find then print("Every node list (fly_in, follow, specific_attack or runaway_from) should contain strings, but entry '" .. _ .. "' is a " .. type(str) .. ".") end
+			if not str.find then goto continue end
 
 			if str then
 				if str:find("group:") then
@@ -506,7 +508,7 @@ local function line_of_sight(self, pos1, pos2, stepsize)
 	-- It continues to advance in the line of sight in search of a real
 	-- obstruction which counts as 'walkable' nodebox.
 	while minetest.registered_nodes[nn]
-	and minetest.registered_nodes[nn].walkable == false do
+	and (minetest.registered_nodes[nn].walkable == false) do
 
 		-- Check if you can still move forward
 		if td < ad + stepsize then
@@ -541,6 +543,53 @@ local function line_of_sight(self, pos1, pos2, stepsize)
 end
 
 
+-- check line of sight (by BrunoMine, tweaked by Astrobe)
+local function new_line_of_sight(self, pos1, pos2, stepsize)
+
+	if not pos1 or not pos2 then return end
+
+	stepsize = stepsize or 1
+
+	local stepv = vmultiply(vdirection(pos1, pos2), stepsize)
+
+	local s, pos = minetest.line_of_sight(pos1, pos2, stepsize)
+
+	-- normal walking and flying mobs can see you through air
+	if s == true then return true end
+
+	-- New pos1 to be analyzed
+	local npos1 = {x = pos1.x, y = pos1.y, z = pos1.z}
+
+	local r, pos = minetest.line_of_sight(npos1, pos2, stepsize)
+
+	-- Checks the return
+	if r == true then return true end
+
+	-- Nodename found
+	local nn = minetest.get_node(pos).name
+
+	-- It continues to advance in the line of sight in search of a real
+	-- obstruction which counts as 'walkable' nodebox.
+	while minetest.registered_nodes[nn]
+	and (minetest.registered_nodes[nn].walkable == false) do
+
+		npos1 = vadd(npos1, stepv)
+
+		if get_distance(npos1, pos2) < stepsize then return true end
+
+		-- scan again
+		r, pos = minetest.line_of_sight(npos1, pos2, stepsize)
+
+		if r == true then return true end
+
+		-- New Nodename found
+		nn = minetest.get_node(pos).name
+	end
+
+	return false
+end
+
+
 -- check line of sight using raycasting (thanks Astrobe)
 local function ray_line_of_sight(self, pos1, pos2)
 
@@ -879,6 +928,10 @@ local function remove_mob(self, decrease)
 
 	if decrease and active_limit > 0 then
 		active_mobs = active_mobs - 1
+
+		if active_mobs < 0 then
+			active_mobs = 0
+		end
 	end
 --print("-- active mobs: " .. active_mobs .. " / " .. active_limit)
 end
@@ -1087,6 +1140,11 @@ end
 -- environmental damage (water, lava, fire, light etc.)
 function mob_class:do_env_damage()
 
+	-- feed/tame text timer (so mob 'full' messages dont spam chat)
+	if self.htimer > 0 then
+		self.htimer = self.htimer - 1
+	end
+
 	self:update_tag()
 
 	local pos = self.object:get_pos() ; if not pos then return end
@@ -1926,9 +1984,7 @@ end
 local function is_peaceful_player(player)
 
 	-- main setting enabled
-	if peaceful_player_enabled then
+	if peaceful_player_enabled and player:is_player() then
-		return true
-	end
 
 		local player_name = player:get_player_name()
 
@@ -1937,6 +1993,7 @@ local function is_peaceful_player(player)
 		and minetest.check_player_privs(player_name, "peaceful_player") then
 			return true
 		end
+	end
 
 	return false
 end
@@ -2126,16 +2183,17 @@ function mob_class:follow_flop()
 
 		for n = 1, #players do
 
-			if players[n]
+			if players[n] then
-			and not is_invisible(self, players[n]:get_player_name())
+				if players[n]:is_player() then
+					if not is_invisible(self, players[n]:get_player_name())
 					and get_distance(players[n]:get_pos(), s) < self.view_range then
-
 						self.following = players[n]
-
 						break
 					end
 				end
 			end
+		end
+	end
 
 	if self.type == "npc"
 	and self.order == "follow"