HPCC-14835 Can't pass a dataset that includes a set/child dataset to Python

Was incorrectly adding the child object to args BEFORE popping the args
stack.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>

plugins/pyembed/pyembed.cpp

@@ -1046,8 +1046,9 @@ protected:
     }
     void pop()
     {
-        addArg(args.getClear());
+        OwnedPyObject arg = args.getClear();
         args.setown((PyObject *) stack.popGet());
+        addArg(arg.getClear());
     }
     void addArg(PyObject *arg)
     {