This avoids buffer overruns on loading and saving of games.
This should fix some crashes.

See Wargus#610

It fixes e.g. this valgrind warning (at loading):

==24878== Invalid write of size 1
==24878==    at 0x3DBC63: PathFinderOutput::Load(lua_State*) (script_unit.cpp:261)
==24878==    by 0x3DD5D5: CclUnit(lua_State*) (script_unit.cpp:500)
==24878==    by 0x48A9F4D: luaD_precall (ldo.c:320)
==24878==    by 0x48BBBB3: luaV_execute (lvm.c:591)
==24878==    by 0x48AA5FC: luaD_call (ldo.c:378)
==24878==    by 0x48A98EA: luaD_rawrunprotected (ldo.c:116)
==24878==    by 0x48AA79C: luaD_pcall (ldo.c:464)
==24878==    by 0x48A1E67: lua_pcall (lapi.c:821)
==24878==    by 0x390DDB: LuaCall(lua_State*, int, int, int, bool) (script.cpp:200)
==24878==    by 0x390D69: LuaCall(int, int, bool) (script.cpp:172)
==24878==    by 0x3914BB: LuaLoadFile(std::filesystem::__cxx11::path const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool) (script.cpp:271)
==24878==    by 0x2B1946: LoadGame(std::filesystem::__cxx11::path const&) (loadgame.cpp:204)
==24878==  Address 0x10819890 is 0 bytes after a block of size 64 alloc'd
==24878==    at 0x483EEDD: operator new(unsigned long) (vg_replace_malloc.c:485)
==24878==    by 0x1D528D: std::__detail::_MakeUniq<PathFinderData>::__single_object std::make_unique<PathFinderData>() (unique_ptr.h:1070)
==24878==    by 0x1CAC05: CUnit::Init() (unit.cpp:425)
==24878==    by 0x1EA254: CUnit::CUnit() (unit.h:142)
==24878==    by 0x1E9AEE: CUnitManager::Load(lua_State*) (unit_manager.cpp:180)
==24878==    by 0x3E287A: CclSlotUsage(lua_State*) (script_unit.cpp:1486)
==24878==    by 0x48A9F4D: luaD_precall (ldo.c:320)
==24878==    by 0x48BBBB3: luaV_execute (lvm.c:591)
==24878==    by 0x48AA5FC: luaD_call (ldo.c:378)
==24878==    by 0x48A98EA: luaD_rawrunprotected (ldo.c:116)
==24878==    by 0x48AA79C: luaD_pcall (ldo.c:464)
==24878==    by 0x48A1E67: lua_pcall (lapi.c:821)