SELECT UNION writes garbage tail of VARCHAR column into internal temporary table

innodb_fts.fulltext_misc 'innodb_plugin' w1 [ fail ]  Found warnings/errors in server log file!

        Test ended at 2017-04-27 13:05:34

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==1037== Thread 22:

==1037== Syscall param write(buf) points to uninitialised byte(s)

==1037==    at 0x4E42B1D: ??? (in /lib64/libpthread-2.18.so)

==1037==    by 0xF1EB0B: my_write (my_write.c:49)

==1037==    by 0xEF31ED: inline_mysql_file_write (mysql_file.h:1131)

==1037==    by 0xEF5E79: _my_b_cache_write (mf_iocache.c:1643)

==1037==    by 0xEF66D4: my_b_flush_io_cache (mf_iocache.c:1843)

==1037==    by 0xEF68C2: end_io_cache (mf_iocache.c:1902)

==1037==    by 0xA8A5A5: maria_extra (ma_extra.c:164)

==1037==    by 0xA4547A: ha_maria::extra(ha_extra_function) (ha_maria.cc:2583)

==1037==    by 0x6F5367: select_union::flush() (sql_union.cc:111)

==1037==    by 0x6F7C07: st_select_lex_unit::exec() (sql_union.cc:849)

==1037==    by 0x6F5045: mysql_union(THD*, LEX*, select_result*, st_select_lex_unit*, unsigned long) (sql_union.cc:39)

==1037==    by 0x658035: handle_select(THD*, LEX*, select_result*, unsigned long) (sql_select.cc:362)

==1037==  Address 0x183197eb is 11 bytes inside a block of size 131,080 alloc'd

==1037==    at 0x4C280F3: malloc (vg_replace_malloc.c:299)

==1037==    by 0xF178DA: my_malloc (my_malloc.c:101)

==1037==    by 0xEF3B76: init_io_cache (mf_iocache.c:272)

==1037==    by 0xA8A512: maria_extra (ma_extra.c:145)

==1037==    by 0xA4547A: ha_maria::extra(ha_extra_function) (ha_maria.cc:2583)

==1037==    by 0x6F5590: select_union::create_result_table(THD*, List<Item>*, bool, unsigned long long, char const*, bool, bool, bool) (sql_union.cc:168)

==1037==    by 0x6F6AFE: st_select_lex_unit::prepare(THD*, select_result*, unsigned long) (sql_union.cc:585)

==1037==    by 0x6F502B: mysql_union(THD*, LEX*, select_result*, st_select_lex_unit*, unsigned long) (sql_union.cc:37)

==1037==    by 0x658035: handle_select(THD*, LEX*, select_result*, unsigned long) (sql_select.cc:362)

==1037==    by 0x627253: execute_sqlcom_select(THD*, TABLE_LIST*) (sql_parse.cc:5920)

==1037==    by 0x61D721: mysql_execute_command(THD*) (sql_parse.cc:2979)

==1037==    by 0x62A952: mysql_parse(THD*, char*, unsigned int, Parser_state*) (sql_parse.cc:7343)