Bugfix: Stratum: When destroying cURL easy handle, be sure to clear pool stratum_curl pointer

util.c

@@ -1567,6 +1567,7 @@ bool initiate_stratum(struct pool *pool)
 	}
 	if (pool->sock == INVSOCK)
 	{
+		pool->stratum_curl = NULL;
 		curl_easy_cleanup(curl);
 		applog(LOG_ERR, "Stratum connect succeeded, but technical problem extracting socket (pool %u)", pool->pool_no);
 		goto out;