Pull a NULL pointer check up to cover more cases in the function.

Found using Clang's static analyzer.
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 0b541ee..fddaed8 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -1833,11 +1833,13 @@
 int
 PySequence_Check(PyObject *s)
 {
-	if (s && PyInstance_Check(s))
+	if (s == NULL)
+		return 0;
+	if (PyInstance_Check(s))
 		return PyObject_HasAttrString(s, "__getitem__");
 	if (PyDict_Check(s))
 		return 0;
-	return s != NULL && s->ob_type->tp_as_sequence &&
+	return  s->ob_type->tp_as_sequence &&
 		s->ob_type->tp_as_sequence->sq_item != NULL;
 }