[Draft] MSAN use-of-uninitialized-value in ha_partition::handle_ordered_prev

LD_LIBRARY_PATH=/home/elenst/msan-libs repro bug.part1 --vardir=/ssd/logs/var-mtr5 --mysqld=--max-allowed-packet=1G --mysqld=--loose-innodb-ft-min-token-size=10 --mysqld=--secure-file-priv= --mysqld=--loose-debug-assert-on-not-freed-memory=1 --mysqld=--binlog-format=row --mysqld=--delay_key_write=OFF --mysqld=--explicit-defaults-for-timestamp=off --mysqld=--innodb-lock-wait-timeout=5 --mysqld=--lock-wait-timeout=10 --mysqld=--log-bin --mysqld=--log-bin-trust-function-creators=1 --mysqld=--log_slow_query_time=3 --mysqld=--max-statement-time=20 --mysqld=--plugin-maturity=experimental --mysqld=--system_versioning_insert_history=ON

==3197211==WARNING: MemorySanitizer: use-of-uninitialized-value

    #0 0x55d910555d08 in ha_partition::handle_ordered_prev(unsigned char*) /data/bld/10.11.15-msan/sql/ha_partition.cc:8476:7

    #1 0x55d90fd16215 in handler::ha_index_prev(unsigned char*) /data/bld/10.11.15-msan/sql/handler.cc:3713:3

    #2 0x55d90f34b8ff in mysql_ha_read(THD*, TABLE_LIST*, enum_ha_read_modes, char const*, List<Item>*, ha_rkey_function, Item*, unsigned long long, unsigned long long) /data/bld/10.11.15-msan/sql/sql_handler.cc:937:29

    #3 0x55d90f433b06 in mysql_execute_command(THD*, bool) /data/bld/10.11.15-msan/sql/sql_parse.cc:5768:10

    #4 0x55d90f421b49 in mysql_parse(THD*, char*, unsigned int, Parser_state*) /data/bld/10.11.15-msan/sql/sql_parse.cc:8186:18

    #5 0x55d90f41acd4 in dispatch_command(enum_server_command, THD*, char*, unsigned int, bool) /data/bld/10.11.15-msan/sql/sql_parse.cc:1906:7

    #6 0x55d90f422c01 in do_command(THD*, bool) /data/bld/10.11.15-msan/sql/sql_parse.cc:1419:17

    #7 0x55d90f89deac in do_handle_one_connection(CONNECT*, bool) /data/bld/10.11.15-msan/sql/sql_connect.cc:1475:11

    #8 0x55d90f89d97a in handle_one_connection /data/bld/10.11.15-msan/sql/sql_connect.cc:1387:5

    #9 0x55d91057c472 in pfs_spawn_thread /data/bld/10.11.15-msan/storage/perfschema/pfs.cc:2201:3

    #10 0x7f3cfb3a91c3 in start_thread nptl/pthread_create.c:442:8

    #11 0x7f3cfb42985b in clone3 misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

  Memory was marked as uninitialized

    #0 0x55d90f0161bd in __msan_allocated_memory (/share8t/bld/10.11.15-msan/sql/mariadbd+0x8661bd) (BuildId: 6586ebee61a73005b63f0d3ba633c19b34bb700d)

    #1 0x55d911178124 in my_malloc /data/bld/10.11.15-msan/mysys/my_malloc.c:116:7

SUMMARY: MemorySanitizer: use-of-uninitialized-value /data/bld/10.11.15-msan/sql/ha_partition.cc:8476:7 in ha_partition::handle_ordered_prev(unsigned char*)

Exiting