MDEV-5521 SET ROLE as prepared statement crashes the server

set_role::light_check() was incorrect

mysql-test/suite/roles/ps.result

+PREPARE stmt FROM 'SET ROLE NONE';

mysql-test/suite/roles/ps.test

+#
+# MDEV-5521 SET ROLE as prepared statement crashes the server
+#
+PREPARE stmt FROM 'SET ROLE NONE';

sql/set_var.cc

@@ -855,9 +855,7 @@ int set_var_password::update(THD *thd)
 int set_var_role::check(THD *thd)
 {
 #ifndef NO_EMBEDDED_ACCESS_CHECKS
-  ulonglong access;
-  int status= acl_check_setrole(thd, base.str, &access);
-  save_result.ulonglong_value= access;
+  int status= acl_check_setrole(thd, role.str, &access);
   return status;
 #else
   return 0;
@@ -867,7 +865,7 @@ int set_var_role::check(THD *thd)
 int set_var_role::update(THD *thd)
 {
 #ifndef NO_EMBEDDED_ACCESS_CHECKS
-  return acl_setrole(thd, base.str, save_result.ulonglong_value);
+  return acl_setrole(thd, role.str, access);
 #else
   return 0;
 #endif

sql/set_var.h

@@ -278,11 +278,12 @@ public:
 
 /* For SET ROLE */
 
-class set_var_role: public set_var
+class set_var_role: public set_var_base
 {
+  LEX_STRING role;
+  ulonglong access;
 public:
-  set_var_role(LEX_STRING role_arg) :
-    set_var(OPT_SESSION, NULL, &role_arg, NULL){};
+  set_var_role(LEX_STRING role_arg) : role(role_arg) {}
   int check(THD *thd);
   int update(THD *thd);
 };