[libc][Obvious] Check if the state hasn't already been destroyed on shutdown

[jhuber6]

Summary

This ensures that if someone calls the rpc_shutdown method multiple
times it will not segfault and gracefully continue. This was causing
problems in the OpenMP usage. This could point to other issues, but for
now this is a safe fix.

libc/utils/gpu/server/rpc_server.cpp

if (state->reference_count == std::numeric_limits<uint32_t>::max())
     return RPC_STATUS_ERROR;
 
   state->reference_count++;
 
   return RPC_STATUS_SUCCESS;
 }
 
 rpc_status_t rpc_shutdown(void) {
-  if (state->reference_count-- == 1)
+  if (state && state->reference_count-- == 1)
     delete state;
 
   return RPC_STATUS_SUCCESS;
 }
 
 rpc_status_t rpc_server_init(uint32_t device_id, uint64_t num_ports,
                              uint32_t lane_size, rpc_alloc_ty alloc,
                              void *data) {