MINOR: httpclient: don't add body when istlen is empty

Don't try to create a request with a body in httpclient_req_gen() if the
payload ist has a ptr but no len.

Sometimes people have their httpclient stuck because they use an ist
with a data ptr but no len. Check the len so this mistake doesn't block
the client.
diff --git a/src/http_client.c b/src/http_client.c
index b336640..bd39823 100644
--- a/src/http_client.c
+++ b/src/http_client.c
@@ -343,7 +343,7 @@
 	if (!htx_add_endof(htx, HTX_BLK_EOH))
 		goto error;
 
-	if (isttest(payload)) {
+	if (isttest(payload) && istlen(payload)) {
 		/* add the payload if it can feat in the buffer, no need to set
 		 * the Content-Length, the data will be sent chunked */
 		if (!htx_add_data_atonce(htx, payload))