fancy-prompt.lua: workaround a Readline UTF-8 bug

plugins/degesch/fancy-prompt.lua

@@ -64,6 +64,10 @@ degesch.hook_prompt (function (hook)
 	local lines, cols = degesch.get_screen_size ()
 	x = x .. " " .. active .. string.rep (" ", cols)
 
+	-- Readline seems to be broken and completely corrupts the prompt
+	-- (tested on 7.0.003 Archlinux, 7.0-5 Debian buster)
+	x = x:gsub("[\128-\255]", "?")
+
 	-- Cut off extra characters and apply formatting, including the hack.
 	-- Note that this doesn't count with full-width or zero-width characters.
 	local overflow = utf8.offset (x, cols - 1)