AP_Scripting: ensure new user data is zero

this prevents use of uninitialised data for user objects created in
lua, giving more predictable behaviour

libraries/AP_Scripting/lua/src/lapi.c

@@ -1189,7 +1189,9 @@ LUA_API void *lua_newuserdata (lua_State *L, size_t size) {
   api_incr_top(L);
   luaC_checkGC(L);
   lua_unlock(L);
-  return getudatamem(u);
+  void *udata = getudatamem(u);
+  memset(udata, 0, size);
+  return udata;
 }