vim-patch:9.0.0009: going past the end of a menu item with only modifier

Problem:    Going past the end of a menu item with only modifier.
Solution:   Check for NUL.
083692d598

src/nvim/message.c

@@ -1748,8 +1748,8 @@ const char *str2special(const char **const sp, const bool replace_spaces, const
       *sp = str + 1;
     }
   } else {
-    // single-byte character or illegal byte
-    *sp = str + 1;
+    // single-byte character, NUL or illegal byte
+    *sp = str + (*str == NUL ? 0 : 1);
   }
 
   // Make special keys and C0 control characters in <> form, also <M-Space>.

src/nvim/testdir/test_menu.vim

@@ -530,4 +530,17 @@ func Test_tmenu()
   tunmenu Test
 endfunc
 
+func Test_only_modifier()
+  exe "tmenu a.b \x80\xfc0"
+  let exp =<< trim [TEXT]
+  --- Menus ---
+  500 a
+    500 b
+        t  - <T-2-^@>
+  [TEXT]
+  call assert_equal(exp, split(execute('tmenu'), "\n"))
+
+  tunmenu a.b
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab