Only curl easy cleanup a stratum curl if it exists.

util.c

@@ -1508,15 +1508,17 @@ void suspend_stratum(struct pool *pool)
 	applog(LOG_INFO, "Closing socket for stratum pool %d", pool->pool_no);
 	mutex_lock(&pool->stratum_lock);
 	pool->stratum_active = pool->stratum_notify = false;
+	if (pool->stratum_curl) {
 #if CURL_HAS_KEEPALIVE
-	curl_easy_cleanup(pool->stratum_curl);
+		curl_easy_cleanup(pool->stratum_curl);
 #else
-	/* Old versions of libcurl seem to crash occasionally on this since
-	 * the socket is modified in keep_sockalive in ways curl does not
-	 * know about so sacrifice the ram knowing we leak one curl handle
-	 * every time we disconnect stratum. */
-	CLOSESOCKET(pool->sock);
+		/* Old versions of libcurl seem to crash occasionally on this since
+		* the socket is modified in keep_sockalive in ways curl does not
+		* know about so sacrifice the ram knowing we leak one curl handle
+		* every time we disconnect stratum. */
+		CLOSESOCKET(pool->sock);
 #endif
+	}
 	pool->stratum_curl = NULL;
 	mutex_unlock(&pool->stratum_lock);
 }