Fix a couple of warnings on Mac OS X.

Mac/Modules/Nav.c

@@ -131,7 +131,7 @@ filldialogoptions(PyObject *d,
 		OSType *fileTypeP,
 		OSType *fileCreatorP)
 {
-	int pos = 0;
+	Py_ssize_t pos = 0;
 	PyObject *key, *value;
 	char *keystr;
 	AEDesc *defaultLocation_storage;

Mac/Modules/cf/pycfbridge.c

@@ -283,7 +283,7 @@ PyCF_Python2CF_string(PyObject *src, CFStringRef *dst) {
 	
 	if (PyString_Check(src)) {
 		if (!PyArg_Parse(src, "es", "ascii", &chars))
-			return NULL; /* This error is more descriptive than the general one below */
+			return 0; /* This error is more descriptive than the general one below */
 		*dst = CFStringCreateWithCString((CFAllocatorRef)NULL, chars, kCFStringEncodingASCII);
 		return 1;
 	}