Fix a couple of warnings on Mac OS X.
diff --git a/Mac/Modules/Nav.c b/Mac/Modules/Nav.c
index 0041f76..6786ea0 100644
--- a/Mac/Modules/Nav.c
+++ b/Mac/Modules/Nav.c
@@ -131,7 +131,7 @@
 		OSType *fileTypeP,
 		OSType *fileCreatorP)
 {
-	int pos = 0;
+	Py_ssize_t pos = 0;
 	PyObject *key, *value;
 	char *keystr;
 	AEDesc *defaultLocation_storage;