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Restore cur_L for specific Lua/C API use case.

Thanks to Peter Cawley.

(cherry-picked from commit e86990f7f)

Consider the following Lua C API function:

```
static int error_after_coroutine_return(lua_State *L)
{
	lua_State *innerL = lua_newthread(L);
	luaL_loadstring(innerL, "print('inner coro')");
	lua_pcall(innerL, 0, 0, 0);
	luaL_error(L, "my fancy error");
	return 0;
}
```

And the following Lua script:
```
local libcur_L = require('libcur_L')

local function onesnap_f(var)
  if var then
    return 1
  else
    return 0
  end
end

-- Compile function to trace with snapshot.
if jit then jit.opt.start('hotloop=1') end
onesnap_f(true)
onesnap_f(true)

local r, s = pcall(libcur_L.error_after_coroutine_return)
onesnap_f(false)
```

This is the only case when `cur_L` is not restored, according to
the analysis done in lj-1066.

This patch changes the error-catching routine, so now the patch
sets the actual cur_L there.
Now it is possible to throw errors on non-executing coroutines,
which is a violation of the Lua C API. So, even though it is now
possible, that behavior should be avoided anyway.

Maxim Kokryashkin:
* added the description and the test for the problem

Resolves tarantool/tarantool#6323

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