perf python: Reference Py_None before returning it

Python None objects are handled just like all the other objects with
respect to their reference counting. Before returning Py_None, its
reference count thus needs to be bumped.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Machata <petrm@mellanox.com>
Link: http://lkml.kernel.org/r/b1e565ecccf68064d8d54f37db5d028dda8fa522.1521675563.git.petrm@mellanox.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/python.c

@@ -1004,8 +1004,10 @@ static PyObject *pyrf_evlist__read_on_cpu(struct pyrf_evlist *pevlist,
 			return PyErr_NoMemory();
 
 		evsel = perf_evlist__event2evsel(evlist, event);
-		if (!evsel)
+		if (!evsel) {
+			Py_INCREF(Py_None);
 			return Py_None;
+		}
 
 		pevent->evsel = evsel;