null pointer dereference if threadpool initialization fails

In function 'threadpool_register_reply_event',
    inlined from 'cpuworker_init' at src/core/mainloop/cpuworker.c:140:13,
    inlined from 'run_tor_main_loop' at src/app/main/main.c:1230:3,
    inlined from 'tor_run_main' at src/app/main/main.c:1359:14,
    inlined from 'tor_main' at src/feature/api/tor_api.c:166:12,
    inlined from 'main' at src/app/main/tor_main.c:32:7:
src/lib/evloop/workqueue.c:631:9: warning: potential null pointer dereference [-Wnull-dereference]
  631 |   if (tp->reply_event) {
      |         ^

if threadpool_new fails, then tp will be null. spawn_func should not normally fail, and furthermore the result will most likely be a non-exploitable segmentation fault, but it is still technically undefined behavior and should be fixed.