Fix NULL error

[Astralcircle]

It seems that a simple check against the table is not enough, so the entity is sometimes NULL

[wire-master] gamemodes/sandbox/entities/entities/base_gmodentity.lua:20: Tried to use a NULL entity!
1. GetPos - [C]:-1
 2. BeingLookedAtByLocalPlayer - gamemodes/sandbox/entities/entities/base_gmodentity.lua:20
  3. BeingLookedAtByLocalPlayer - addons/wire-master/lua/entities/base_wire_entity.lua:230
   4. fn - addons/wire-master/lua/entities/base_wire_entity.lua:265
    5. unknown - addons/ulib-master/lua/ulib/shared/hook.lua:109