Check if mining threads are initialized before restarting them.

sgminer.c

@@ -4082,6 +4082,7 @@ static void *restart_thread(void __maybe_unused *arg)
   /* Discard staged work that is now stale */
   discard_stale();
 
+  if (mining_thr) {
     rd_lock(&mining_thr_lock);
     for (i = 0; i < mining_threads; i++) {
       cgpu = mining_thr[i]->cgpu;
@@ -4093,6 +4094,7 @@ static void *restart_thread(void __maybe_unused *arg)
       cgpu->drv->flush_work(cgpu);
     }
     rd_unlock(&mining_thr_lock);
+  }
 
   mutex_lock(&restart_lock);
   pthread_cond_broadcast(&restart_cond);