unit-tests: Zero-initialize chunk to avoid free on non-successful fetch

If the fetch fails, the fetcher is not required to return an empty chunk. Avoid
the resulting invalid free() by initializing data.ptr to NULL.

src/libstrongswan/tests/suites/test_fetch_http.c

@@ -281,7 +281,7 @@ START_TEST(test_response_code)
 {
 	stream_service_t *service;
 	status_t status;
-	chunk_t data;
+	chunk_t data = chunk_empty;
 	char uri[256];
 	u_int code;