fix another source of problems with 7.8 compat SSL testsuite

we can't assume that because the send buffer is empty, no more data
can be expected. should still identify problems, as test will either
succeed or fail by the backend countdown timing out.

lib/7.8/modules/SSL.pmod/testsuite.in

@@ -99,13 +99,16 @@ define(test_ssl, [[
 
     int trigged = 15;
 
+    int ssd = 0;
+
     string server_buf = "";
     void server_send_data()
     {
       trigged = 10;
       int bytes = server->write(server_buf);
+      ssd += bytes;
       server_buf = server_buf[bytes..];
-      if (!sizeof(server_buf)) {
+      if (!sizeof(server_buf) && ssd == sizeof($5)) {
         server->set_write_callback(UNDEFINED);
         if (state) {
           server->close();