If a meshnode controller was trying to make sense of metadata from an
item other than meshnode glue, the server would crash. To prevent this,
only meshnode glue is allowed to be in a meshnode controller inventory.
Previously, any player attached to the meshnode controller was floating
above it. This could lead to the attached player being unable to detach
from the controller, as a node could fit between player and controller.
I disabled some mods and a ship produces crash with error
```
2018-01-10 16:55:16: ERROR[Main]: ServerError: AsyncErr: ServerThread::run Lua: Runtime error from mod 'meshnode' in callback luaentity_Activate(): /usr/share/minetest/builtin/common/misc_helpers.lua:590: table index is nil
2018-01-10 16:55:16: ERROR[Main]: stack traceback:
2018-01-10 16:55:16: ERROR[Main]: /usr/share/minetest/builtin/common/misc_helpers.lua:590: in function 'copy'
2018-01-10 16:55:16: ERROR[Main]: .../games/whynot/mods/tools/meshnode/api.lua:40: in function 'get_tile_textures'
2018-01-10 16:55:16: ERROR[Main]: .../games/whynot/mods/tools/meshnode/api.lua:205: in function 'add_entity'
2018-01-10 16:55:16: ERROR[Main]: .../games/whynot/mods/tools/meshnode/init.lua:192: in function <.../games/whynot/mods/tools/meshnode/init.lua:160>
```
So this is a quick fix to get the world working again