lua_npge.cpp: add assert about string length
lua_npge.cpp: add assert about string length

See https://ci.appveyor.com/project/starius/lua-npge/build/0.0.1.81-test/job/qq2s8wlva49808in#L705

warning C4267: 'initializing' : conversion from 'size_t' to 'int', possible loss of data
src/npge/cpp/lua_npge.cpp(474) : see reference to function template instantiation
 'std::pair<const char *,int>::pair<const char*&,size_t&,void>(_Other1,_Other2)'
 being compiled
with
[
    _Other1=const char *&
,            _Other2=size_t &
]

npge/lua-npge,npge/lua-npge,starius/lua-npge,npge/lua-npge,starius/lua-npge,starius/lua-npge