use context service buffer instead of stack for clent_connect

Signed-off-by: Andy Green <andy.green@linaro.org>
diff --git a/lib/client-handshake.c b/lib/client-handshake.c
index 1fc78f7..9292c03 100644
--- a/lib/client-handshake.c
+++ b/lib/client-handshake.c
@@ -20,7 +20,7 @@
 	 */
 
 	if (context->http_proxy_port) {
-		plen = sprintf(context->service_buffer,
+		plen = sprintf((char *)context->service_buffer,
 			"CONNECT %s:%u HTTP/1.0\x0d\x0a"
 			"User-agent: libwebsockets\x0d\x0a"
 /*Proxy-authorization: basic aGVsbG86d29ybGQ= */