formspec_escape stability improvement

* formspec_escape causes server crash, when text argument is a number.
  the `tostring(text)` prevents this
* backported https://github.com/MultiCraft/MultiCraft/pull/87

builtin/common/misc_helpers.lua

@@ -254,6 +254,7 @@ end
 
 
 function core.formspec_escape(text)
+	text = tostring(text)
 	if text ~= nil then
 		text = text:gsub("\\", "\\\\")
 		text = text:gsub("%]", "\\]")