tests: Fix use-after-free in Python tests

The nodeset must be freed before the document.

Fixes #443.

python/tests/xpathns.py

@@ -11,6 +11,7 @@ d = libxml2.parseDoc("<a:a xmlns:a='urn:whatevar'/>")
 res=""
 for n in d.xpathEval("//namespace::*"):
     res = res + n.serialize()
+del n
 d.freeDoc()
 
 if res != expect:
@@ -18,7 +19,6 @@ if res != expect:
     print(res)
 del res
 del d
-del n
 # Memory debug specific
 libxml2.cleanupParser()