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

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.

Differential Revision: https://reviews.llvm.org/D155005

Author: jhuber6

libc/utils/gpu/server/rpc_server.cpp

@@ -231,7 +231,7 @@ rpc_status_t rpc_init(uint32_t num_devices) {
 }
 
 rpc_status_t rpc_shutdown(void) {
-  if (state->reference_count-- == 1)
+  if (state && state->reference_count-- == 1)
     delete state;
 
   return RPC_STATUS_SUCCESS;