LeakSanitizer errors in rec_copy_prefix_to_buf

--source include/have_innodb.inc

--source include/have_sequence.inc

CREATE TABLE t (a INT, b VARCHAR(8), FULLTEXT KEY(b))

  ENGINE=InnoDB WITH SYSTEM VERSIONING;

INSERT INTO t (a) SELECT seq FROM seq_1_to_500;

UPDATE t SET a = 0;

ALTER TABLE t FORCE;

DROP TABLE  t;

==92907==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 50 byte(s) in 1 object(s) allocated from:

    #0 0x7fd8f00b89cf in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69

    #1 0x5636173eef15 in ut_allocator<unsigned char, true>::allocate(unsigned long, unsigned char const*, unsigned int, bool, bool) /data/bld/10.11-asan-ubsan/storage/innobase/include/ut0new.h:374

    #2 0x56361788562f in rec_copy_prefix_to_buf(unsigned char const*, dict_index_t const*, unsigned long, unsigned char**, unsigned long*) /data/bld/10.11-asan-ubsan/storage/innobase/rem/rem0rec.cc:2095

    #3 0x563617c50022 in btr_pcur_store_position(btr_pcur_t*, mtr_t*) /data/bld/10.11-asan-ubsan/storage/innobase/btr/btr0pcur.cc:178

    #4 0x563617c5532b in btr_pcur_move_backward_from_page /data/bld/10.11-asan-ubsan/storage/innobase/btr/btr0pcur.cc:580

    #5 0x563617c55e63 in btr_pcur_move_to_prev(btr_pcur_t*, mtr_t*) /data/bld/10.11-asan-ubsan/storage/innobase/btr/btr0pcur.cc:645

    #6 0x563617efca6c in fts_get_max_doc_id(dict_table_t*) /data/bld/10.11-asan-ubsan/storage/innobase/fts/fts0fts.cc:3676

    #7 0x563617949617 in row_merge_build_indexes(trx_t*, dict_table_t*, dict_table_t*, bool, dict_index_t**, unsigned long const*, unsigned long, TABLE*, dtuple_t const*, unsigned long const*, unsigned long, ib_sequence_t&, bool, ut_stage_alter_t*, dict_add_v_col_t const*, TABLE*, bool, std::__debug::map<unsigned int, dict_col_t*, std::less<unsigned int>, std::allocator<std::pair<unsigned int const, dict_col_t*> > > const*) /data/bld/10.11-asan-ubsan/storage/innobase/row/row0merge.cc:4731

    #8 0x56361749b8e9 in ha_innobase::inplace_alter_table(TABLE*, Alter_inplace_info*) /data/bld/10.11-asan-ubsan/storage/innobase/handler/handler0alter.cc:8966

    #9 0x5636152ff8ab in handler::ha_inplace_alter_table(TABLE*, Alter_inplace_info*) /data/bld/10.11-asan-ubsan/sql/handler.h:4625

    #10 0x5636152ff8ab in mysql_inplace_alter_table /data/bld/10.11-asan-ubsan/sql/sql_table.cc:8133

    #11 0x563615377cd2 in mysql_alter_table(THD*, st_mysql_const_lex_string const*, st_mysql_const_lex_string const*, Table_specification_st*, TABLE_LIST*, Recreate_info*, Alter_info*, unsigned int, st_order*, bool, bool) /data/bld/10.11-asan-ubsan/sql/sql_table.cc:11467

    #12 0x56361568bdbb in Sql_cmd_alter_table::execute(THD*) /data/bld/10.11-asan-ubsan/sql/sql_alter.cc:688

    #13 0x563614e4b96c in mysql_execute_command(THD*, bool) /data/bld/10.11-asan-ubsan/sql/sql_parse.cc:6201

    #14 0x563614e501b7 in mysql_parse(THD*, char*, unsigned int, Parser_state*) /data/bld/10.11-asan-ubsan/sql/sql_parse.cc:8223

    #15 0x563614e59529 in dispatch_command(enum_server_command, THD*, char*, unsigned int, bool) /data/bld/10.11-asan-ubsan/sql/sql_parse.cc:1924

    #16 0x563614e65bd6 in do_command(THD*, bool) /data/bld/10.11-asan-ubsan/sql/sql_parse.cc:1434

    #17 0x56361565b7fd in do_handle_one_connection(CONNECT*, bool) /data/bld/10.11-asan-ubsan/sql/sql_connect.cc:1475

    #18 0x56361565c95a in handle_one_connection /data/bld/10.11-asan-ubsan/sql/sql_connect.cc:1387

    #19 0x563616f932b0 in pfs_spawn_thread /data/bld/10.11-asan-ubsan/storage/perfschema/pfs.cc:2201

    #20 0x7fd8eeca81c3 in start_thread nptl/pthread_create.c:442

SUMMARY: AddressSanitizer: 50 byte(s) leaked in 1 allocation(s).