This fixes this error:
```
2021-10-02 19:43:43: ERROR[Main]: ServerError: AsyncErr: ServerThread::run Lua: Runtime error from mod 'mobs_turtles' in callback luaentity_run_simple_callback(): ...test-pvp/.minetest/mods/mobs_water/mobs_turtles/init.lua:54: attempt to call global 'set_velocity' (a nil value)
2021-10-02 19:43:43: ERROR[Main]: stack traceback:
2021-10-02 19:43:43: ERROR[Main]: ...test-pvp/.minetest/mods/mobs_water/mobs_turtles/init.lua:54: in function 'func'
2021-10-02 19:43:43: ERROR[Main]: /usr/share/minetest/builtin/profiler/instrumentation.lua:106: in function </usr/share/minetest/builtin/profiler/instrumentation.lua:100>
```
