Fix server crash by client leaving after joining

When a player joins and immediately leaves the game before a function is
called by minetest.after() in mods/PLAYER/wieldview/init.lua, it gets an
invalidated player object. This results in the player methods returning
nil (since Minetest 5.2); perhaps surprisingly, the player is not nil.

Not checking that the result of player:get_pos() is not nil could lead
to a server crash if a client crashed when joining. It has been reported
that a syntax error in a client side mod was enough to trigger that.

mods/PLAYER/wieldview/init.lua

@@ -70,8 +70,22 @@ minetest.register_on_joinplayer(function(player)
 	local name = player:get_player_name()
 	wieldview.wielded_item[name] = ""
 	minetest.after(0, function(player)
+		-- When a player joins and immediately leaves the game
+		-- before this function is called by minetest.after(),
+		-- the player object is invalidated. This results in
+		-- its methods returning nil (since Minetest 5.2);
+		-- perhaps surprisingly, the player is not nil.
+		--
+		-- Not checking that the position is not nil can lead
+		-- to a server crash if a client crashes while it is
+		-- joining. It has been reported that a syntax error
+		-- in a client side mod was enough to trigger that.
+		local pos = player:get_pos()
+		if pos == nil then
+			return
+		end
 		wieldview:update_wielded_item(player)
-		local itementity = minetest.add_entity(player:get_pos(), "wieldview:wieldnode")
+		local itementity = minetest.add_entity(pos, "wieldview:wieldnode")
 		itementity:set_attach(player, "Hand_Right", vector.new(0, 0, 0), vector.new(15, 45, 0))
 		itementity:get_luaentity().wielder = name
 	end, player)