Fixed nil value error for ironsights

[mexikoedi]

This PR fixes the following error:

[TTT2 (Base) - v0.14.5b] gamemodes/terrortown/entities/weapons/weapon_tttbase.lua:1676: attempt to call method 'SetIronsights' (a nil value)
  1. fn - gamemodes/terrortown/entities/weapons/weapon_tttbase.lua:1676
   2. unknown - lua/ulib/shared/hook.lua:115

The error appeared after a round ended and when a new round started.
It also makes the code more robust (also for SetZoom).