sharedata got stack overflow when 100w string key in use

--examples/sharedata_size.lua
local skynet = require "skynet"
local sharedata = require "skynet.sharedata"

local big_table = {}
local size = 1000000
for i=1,size do
        big_table[tostring(i)] = i
end

skynet.start(function()
        skynet.error("new foobar")
        sharedata.new("foobar", big_table)
end)

跑这个样例的时候会报错StackOverflow

[:01000004] master listen socket 0.0.0.0:2013
[:01000005] LAUNCH snlua cslave
[:01000005] slave connect to master 127.0.0.1:2013
[:01000004] connect from 127.0.0.1:33644 4
[:01000006] LAUNCH harbor 1 16777221
[:01000004] Harbor 1 (fd=4) report 127.0.0.1:2526
[:01000005] Waiting for 0 harbors
[:01000005] Shakehand ready
[:01000007] LAUNCH snlua datacenterd
[:01000008] LAUNCH snlua service_mgr
[:01000009] LAUNCH snlua sharedata_size
[:01000009] Memory warning 32.00 M
[:01000009] Memory warning 64.00 M
[:0100000a] LAUNCH snlua sharedatad
[:01000009] new foobar
[:0100000a] Memory warning 32.00 M
[:0100000a] Memory warning 64.00 M
[:0100000a] lua call [1000009 to :100000a : 3 msgsz = 11757584] error : ./lualib/skynet.lua:988: ./lualib/skynet.lua:452: stack overflow
stack traceback:
        [C]: in function 'skynet.sharedata.core.new'
        ./service/sharedatad.lua:15: in upvalue 'newobj'
        ./service/sharedatad.lua:76: in local 'f'
        ./service/sharedatad.lua:162: in upvalue 'f'
        ./lualib/skynet.lua:402: in function <./lualib/skynet.lua:374>
stack traceback:
        [C]: in function 'assert'
        ./lualib/skynet.lua:988: in function 'skynet.dispatch_message'
[:01000009] init service failed: ./lualib/skynet.lua:720: call failed
stack traceback:
        [C]: in function 'error'
        ./lualib/skynet.lua:720: in upvalue 'yield_call'
        ./lualib/skynet.lua:737: in function 'skynet.call'
        ./lualib/skynet/sharedata.lua:45: in function 'skynet.sharedata.new'
        ./examples/sharedata_size.lua:12: in upvalue 'start'
        ./lualib/skynet.lua:1065: in function <./lualib/skynet.lua:1063>
        [C]: in function 'xpcall'
        ./lualib/skynet.lua:1067: in function 'skynet.init_service'
        ./lualib/skynet.lua:1080: in upvalue 'f'
        ./lualib/skynet.lua:375: in function <./lualib/skynet.lua:374>
[:01000009] KILL self
[:01000002] KILL self

注释掉 lualib-src/lua-sharedata.c:373能好，luaL_checkstack应该在使用lua栈之前用；但是有同学觉得lualib-src/lua-sharedata.c:383的lua_settop会导致栈顶设置异常，大家有什么建议呢？

Start with examples/sharedata_size.lua and reproduce the stack overflow using the million-string-key table. Then inspect lualib-src/lua-sharedata.c around lines 373 and 383, focusing on luaL_checkstack and lua_settop. Done means the example completes without stack overflow and the sharedata stack handling is verified.