Fix crash bug when calling cpuworkers_rotate_keyinfo on a client.

Fixes bug 15245; bugfix on 0.2.6.3-alpha. Thanks to anonym for reporting!

changes/bug15245

@@ -0,0 +1,5 @@
+  o Major bugfixes:
+    - Avoid crashing when making certain configuration option changes
+      on clients. Fixes bug 15245; bugfix on 0.2.6.3-alpha. Reported
+      by "anonym".
+

src/or/cpuworker.c

@@ -178,6 +178,8 @@ update_state_threadfn(void *state_, void *work_)
 void
 cpuworkers_rotate_keyinfo(void)
 {
+  if (!threadpool)
+    return;
   if (threadpool_queue_update(threadpool,
                               worker_state_new,
                               update_state_threadfn,