Add null pointer safety to fx_bonusfire

[squeek502]

The entity sent over the network in the effect data could potentially not exist once it reaches the client, which would cause a null pointer crash

Contributes to #311

[squeek502]

Confirmed that this can occur:

• Added this code: engine->ServerCommand(UTIL_VarArgs("kick %s\n", this->GetPlayerName())); right after DispatchEvent("BonusFire", data) in ff_player.cpp
• Set net_fakelag to 200 (not sure if this is needed)
• Added a bot, set him on fire and shot him with an IC to trigger the bonus fire effect
• Bot gets kicked, ent index is received on the client and assumed to exist -> null pointer crash

Going to merge and release another hotfix since we know this is another crash.