Follow up commit for wl#9306, fix for compilation warnings on OSX

Description:
OSX doesn't detect properly that "msgid" variable is properly
initialized in one of sub-calls.

Solution:
Changed the source code to be better understandable for the
compiler, from:
  while(!stop) { ... }
to:
  do { ... } while(!stop);

Reviewed-by: Andrzej Religa <[email protected]>

Author: lkotula

rapid/plugin/x/mysqlxtest_src/mysqlxtest.cc

@@ -1133,15 +1133,15 @@ class Command
 
     Message_by_name::iterator iterator_msg_id = server_msgs_by_name.find(iterator_msg_name->second);
 
-	if (server_msgs_by_name.end() == iterator_msg_id)
+    if (server_msgs_by_name.end() == iterator_msg_id)
     {
       std::cout << "Invalid data in internal message list, entry not found:" << iterator_msg_name->second << "\n";
       return Stop_with_failure;
     }
 
     const int expected_msg_id = iterator_msg_id->second.second;
 
-    while (!stop)
+    do
     {
       boost::scoped_ptr<mysqlx::Message> msg(context.connection()->recv_raw(msgid));
       if (msg.get())
@@ -1166,6 +1166,8 @@ class Command
         }
       }
     }
+    while (!stop);
+
     variables_to_unreplace.clear();
 
     if (Mysqlx::ServerMessages::ERROR == msgid &&