patch 8.2.3771: Vim9: accessing freed memory when checking type

Problem:    Vim9: accessing freed memory when checking type.
Solution:   Make a copy of a function type.

src/structs.h

@@ -1809,6 +1809,7 @@ struct svar_S {
     char_u	*sv_name;	// points into "sn_all_vars" di_key
     typval_T	*sv_tv;		// points into "sn_vars" or "sn_all_vars" di_tv
     type_T	*sv_type;
+    int		sv_type_allocated;  // call free_type() for sv_type
     int		sv_const;	// 0, ASSIGN_CONST or ASSIGN_FINAL
     int		sv_export;	// "export let var = val"
 };