Fix SV_SetClient_Ping crash due to unlinked client pointers

[google-labs-jules]

Fixes a crash in SV_SetClient_Ping caused by client->edict->client being NULL. This happens if the engine iterates over clients (e.g. for ping updates) before the game DLL has fully connected the client to the entity.

Changes:

• Modified AllocateClientArray in src/server/gameplay/g_clients.cpp to initialize g_entities[i+1].client for all allocated clients immediately.
• Modified FreeClientArray in src/server/gameplay/g_clients.cpp to clear g_entities[i+1].client pointers before freeing memory.

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PR created automatically by Jules for task 6581969964319597072 started by @themuffinator

[google-labs-jules]

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