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LJ_GC64: Fix lua_concat().

Reported by Mathias Westerdahl.

(cherry picked from commit 633f265f6)

Lua 5.1 Reference Manual [1] defines a function `lua_concat`, that:

> void lua_concat (lua_State *L, int n);
>
> Concatenates the n values at the top of the stack, pops them, and leaves
> the result at the top.

Without the patch lua_concat() behaved incorrectly with userdata.
Assuming we have three literals and a userdata with defined "__concat"
metamethod on top of the Lua stack:

1 [string]
2 [string]
3 [string]
4 [string]
5 [userdata] <--- top

On attempt to concatenate *two* items, lua_concat() concatenates *four*
items and leaves result at the top:

1 [string]
2 [string]
3 [string][string][string][userdata] <--- top

The patch fixes this behaviour.

1. https://www.lua.org/manual/5.1/manual.html

Sergey Bronnikov:
  * added the description and the test for the problem

Part of tarantool/tarantool#8825

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