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.

src/message.c

@@ -1820,8 +1820,8 @@ str2special(
 	    *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>.
     // Use <Space> only for lhs of a mapping.

src/testdir/test_menu.vim

@@ -528,4 +528,17 @@ 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

src/version.c

@@ -735,6 +735,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    9,
 /**/
     8,
 /**/