Fixed a potential null pointer dereference and a case of undefined behaviour

src/common/runtime.cpp

@@ -538,7 +538,7 @@ int luax_insist(lua_State *L, int idx, const char *k)
 {
 	// Convert to absolute index if necessary.
 	if (idx < 0 && idx > LUA_REGISTRYINDEX)
-		idx = lua_gettop(L) + ++idx;
+		idx += lua_gettop(L) + 1;
 
 	lua_getfield(L, idx, k);