patch 8.2.1674: Vim9: internal error when using variable that was not set

Problem:    Vim9: internal error when using variable that was not set.
Solution:   Give a meaningful error. (closes #6937)

src/testdir/test_vim9_script.vim

@@ -3308,6 +3308,14 @@ def Test_invalid_sid()
   delete('Xdidit')
 enddef
 
+def Test_unset_any_variable()
+  let lines =<< trim END
+    let var: any
+    assert_equal(0, var)
+  END
+  CheckDefAndScriptSuccess(lines)
+enddef
+
 " Keep this last, it messes up highlighting.
 def Test_substitute_cmd()
   new

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1674,
 /**/
     1673,
 /**/

src/vim9script.c

@@ -548,7 +548,11 @@ vim9_declare_scriptvar(exarg_T *eap, char_u *arg)
 
     // Create the variable with 0/NULL value.
     CLEAR_FIELD(init_tv);
-    init_tv.v_type = type->tt_type;
+    if (type->tt_type == VAR_ANY)
+	// A variable of type "any" is not possible, just use zero instead
+	init_tv.v_type = VAR_NUMBER;
+    else
+	init_tv.v_type = type->tt_type;
     set_var_const(name, type, &init_tv, FALSE, 0);
 
     vim_free(name);