MDEV-5238 Server crashes in find_role_grant_pair on SHOW GRANTS for an anonymous user

mysql-test/suite/roles/show_grants_anon-5238.result

+create user ''@localhost;
+create role r1;
+grant r1 to ''@localhost;
+select current_user;
+current_user
+@localhost
+show grants;
+Grants for @localhost
+GRANT r1 TO ''@'localhost'
+GRANT USAGE ON *.* TO ''@'localhost'
+drop role r1;
+drop user ''@localhost;

mysql-test/suite/roles/show_grants_anon-5238.test

+#
+# MDEV-5238 Server crashes in find_role_grant_pair on SHOW GRANTS for an anonymous user
+#
+--source include/not_embedded.inc
+
+create user ''@localhost;
+create role r1;
+grant r1 to ''@localhost;
+
+--connect (con1,localhost,nonexisting_user,,)
+select current_user;
+show grants;
+
+connection default;
+drop role r1;
+drop user ''@localhost;
+

sql/sql_acl.cc

@@ -7556,7 +7556,7 @@ static ROLE_GRANT_PAIR *find_role_grant_pair(const LEX_STRING *u,
   pair_key.alloc(key_length);
 
   strmov(strmov(strmov(const_cast<char*>(pair_key.ptr()),
-                       u->str) + 1, h->str) + 1, r->str);
+                       safe_str(u->str)) + 1, h->str) + 1, r->str);
 
   return (ROLE_GRANT_PAIR *)
     my_hash_search(&acl_roles_mappings, (uchar*)pair_key.ptr(), key_length);