BUG/MINOR: contrib/spoa-server: Do not free reference to NULL

As per https://docs.python.org/3/c-api/refcounting.html, Py_DECREF
should not be called on NULL objects.

This patch must be backported as far as 2.0.
diff --git a/contrib/spoa_server/ps_python.c b/contrib/spoa_server/ps_python.c
index 427ef18..474c75b 100644
--- a/contrib/spoa_server/ps_python.c
+++ b/contrib/spoa_server/ps_python.c
@@ -558,7 +558,6 @@
 		ent = PyDict_New();
 		if (ent == NULL) {
 			Py_DECREF(kw_args);
-			Py_DECREF(ent);
 			PyErr_Print();
 			return 0;
 		}