Initialize values in sha256_password_authenticate

[ 52%] main.plugin_auth_sha256_2                w6 [ fail ]  Found warnings/errors in server log file!
        Test ended at 2020-01-08 11:23:39
line
==30250== Thread 26:
==30250== Conditional jump or move depends on uninitialised value(s)
==30250==    at 0x483BC35: strlen (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==30250==    by 0x17418E6: sha256_password_authenticate(st_plugin_vio*, st_mysql_server_auth_info*) (sql_authentication.cc:3058)
==30250==    by 0x173ECE7: do_auth_once(THD*, st_mysql_const_lex_string const&, MPVIO_EXT*) (sql_authentication.cc:1959)
==30250==    by 0x173F82C: acl_authenticate(THD*, enum_server_command, bool) (sql_authentication.cc:2235)
==30250==    by 0x151E51C: check_connection(THD*, bool) (sql_connect.cc:1307)
==30250==    by 0x151E6B0: login_connection(THD*, bool) (sql_connect.cc:1364)
==30250==    by 0x151EED8: thd_prepare_connection(THD*, bool) (sql_connect.cc:1527)
==30250==    by 0x16C103B: handle_connection (connection_handler_per_thread.cc:319)
==30250==    by 0x19D8459: pfs_spawn_thread (pfs.cc:2198)
==30250==    by 0x485B181: start_thread (pthread_create.c:486)
==30250==    by 0x52B3B1E: clone (clone.S:95)