Crash with mvcc and exclude_null

[alyapunov]

Simple repro:

box.cfg({
  listen='localhost:3333',
  memtx_use_mvcc_engine=true,
})

space = box.schema.space.create('test', { if_not_exists = true })
space:format({
  { name = 'id';    type = 'unsigned';  },
  { name = 'data';  type = 'string';  },
  { name = 'cnt';      type = 'unsigned';  is_nullable=true },
})

space:create_index( 'pk', {
  parts = {'id'},
  if_not_exists = true,
})

space:create_index( 'second', {
  parts = {{'data'}, {'cnt', exclude_null=true}},
  unique = false,
  if_not_exists = true,
})

require('console').start()

Then call box.space.test:insert({1, 'fdf'})

If falls with the following backtrace

#1  0x55cc76b35385 in crash_collect+256
#2  0x55cc76b35d97 in crash_signal_cb+100
#3  0x7ffa72fd9420 in __restore_rt+0
#4  0x55cc768d1585 in tuple_has_flag+56
#5  0x55cc768d7e8d in memtx_tx_handle_gap_write+404
#6  0x55cc768d8960 in memtx_tx_history_add_insert_stmt+1106
#7  0x55cc768d8e92 in memtx_tx_history_add_stmt+287
#8  0x55cc768edb1b in memtx_space_replace_all_keys+361
#9  0x55cc768edd85 in memtx_space_replace_tuple+177
#10 0x55cc768edf44 in memtx_space_execute_replace+180
#11 0x55cc76975ad7 in space_execute_dml+613
#12 0x55cc769c1234 in box_process_rw+306
#13 0x55cc769cdace in box_process1+453
#14 0x55cc769ce5f8 in box_insert+181
#15 0x55cc76acff7f in lbox_insert+307
#16 0x55cc76b9d827 in lj_BC_FUNCC+70
#17 0x55cc76baadc6 in lua_pcall+945
#18 0x55cc76b13df9 in luaT_call+45
#19 0x55cc76b06f02 in lua_main+265
#20 0x55cc76b07acc in run_script_f+2993
#21 0x55cc7688303a in fiber_cxx_invoke(int (*)(__va_list_tag*), __va_list_tag*)+34
#22 0x55cc76b41919 in fiber_loop+210
#23 0x55cc76ee8f6d in coro_init+120
Aborted (core dumped)