Guard against nil text in menu widget

Allow one to go further in the FM's setting menu on legacy devices...
... and potentially shoot oneself in the foot because of the unusable
confirmdialogs (among other things ;p).

frontend/ui/widget/menu.lua

@@ -166,8 +166,9 @@ function MenuItem:init()
                     ""..mandatory, true, self.bold).x
 
     local state_button_width = self.state_size.w or 0
+    local my_text = self.text and ""..self.text or ""
     w = RenderText:sizeUtf8Text(0, self.dimen.w, self.face,
-                    self.text, true, self.bold).x
+                    ""..my_text, true, self.bold).x
     if w + mandatory_w + state_button_width >= self.content_width then
         if Device:hasKeyboard() then
             self.active_key_events.ShowItemDetail = {
@@ -177,7 +178,7 @@ function MenuItem:init()
         local indicator = "  >> "
         local indicator_w = RenderText:sizeUtf8Text(0, self.dimen.w, self.face,
                         indicator, true, self.bold).x
-        self.text = RenderText:getSubTextByWidth(self.text, self.face,
+        self.text = RenderText:getSubTextByWidth(my_text, self.face,
             self.content_width - indicator_w - mandatory_w - state_button_width,
             true, self.bold) .. indicator
     end