Escape special characters when searching the server list

* fix issue from https://github.com/minetest/minetest/issues/7857
* Simple fix to stop searching for "[" or other special characters causing a crash.

builtin/mainmenu/tab_online.lua

@@ -254,6 +254,7 @@ local function main_button_handler(tabview, fields, name, tabdata)
 		-- setup the keyword list
 		local keywords = {}
 		for word in input:gmatch("%S+") do
+			word = word:gsub("(%W)", "%%%1")
 			table.insert(keywords, word)
 		end