Fix poorly thought-through code from commit 5c3c3cd0a3046339.

It's not entirely clear to me whether PyString_AsString can return
null (looks like the answer might vary between Python 2 and 3).
But in any case, this code's attempt to cope with the possibility
was quite broken, because pstrdup() neither allows a null argument
nor ever returns a null.

Moreover, the code below this point assumes that "message" is a
palloc'd string, which would not be the case for a dgettext result.

Fix both problems by doing the pstrdup step separately.

src/pl/plpython/plpy_plpymodule.c

@@ -424,11 +424,12 @@ PLy_output(volatile int level, PyObject *self, PyObject *args, PyObject *kw)
 	else
 		so = PyObject_Str(args);
 
-	if (so == NULL || ((message = pstrdup(PyString_AsString(so))) == NULL))
+	if (so == NULL || ((message = PyString_AsString(so)) == NULL))
 	{
 		level = ERROR;
 		message = dgettext(TEXTDOMAIN, "could not parse error message in plpy.elog");
 	}
+	message = pstrdup(message);
 
 	Py_XDECREF(so);
 
@@ -444,6 +445,7 @@ PLy_output(volatile int level, PyObject *self, PyObject *args, PyObject *kw)
 				if (PyTuple_Size(args) != 0)
 					PLy_elog(ERROR, "the message is already specified");
 
+				if (message)
 					pfree(message);
 				message = object_to_string(value);
 			}