patch 9.0.0959: error when using the "File Settings / Text Width" menu

Problem:    Error when using the "File Settings / Text Width" menu.
Solution:   Use str2nr(). (closes #11624)
diff --git a/runtime/menu.vim b/runtime/menu.vim
index d449a75..ab509a6 100644
--- a/runtime/menu.vim
+++ b/runtime/menu.vim
@@ -341,7 +341,7 @@
     # Remove leading zeros to avoid it being used as an octal number.
     # But keep a zero by itself.
     var tw = substitute(n, "^0*", "", "")
-    &tw = tw == '' ? 0 : tw
+    &tw = tw == '' ? 0 : str2nr(tw)
   endif
 enddef