Fixed GetHeadshotMultiplier nil value error

[mexikoedi]

This PR fixes the following error:

[TTT2 (Base) - v0.14.5b] gamemodes/terrortown/gamemode/server/sv_player.lua:1113: attempt to call method 'GetHeadshotMultiplier' (a nil value)
  1. unknown - gamemodes/terrortown/gamemode/server/sv_player.lua:1113

The error appeared on this map: [TTT] ttt_terminus_v1

If you are a Traitor you can call combine policemen.
Some of them have pistols and these are not compatible with TTT/TTT2.
You get this error if you try to pick up the pistol:

[TTT2 (Base) - v0.14.5b] Equipped weapon weapon_pistol is not compatible with TTT
  1. unknown - gamemodes/terrortown/gamemode/server/sv_weaponry.lua:529

This is correct because you shouldn't be able to pick up weapons which aren't compatible.

But if they hit you in the head with the pistol it causes the above GetHeadshotMultiplier error which shouldn't happen.

I tested this PR and the combine policemen didn't cause this error anymore if the hit the head of a player.

[saibotk]

Thanka for the fix that looks reasonable