Fixed race condition in RandomX thread init

Thread could deadlock if it started before dataset struct was allocated.

src/backend/cpu/CpuWorker.cpp

@@ -90,6 +90,8 @@ void xmrig::CpuWorker<N>::allocateRandomX_VM()
         if (Nonce::sequence(Nonce::CPU) == 0) {
             return;
         }
+
+        dataset = Rx::dataset(m_job.currentJob(), m_node);
     }
 
     if (!m_vm) {