Fix printing of time_t values set in the distant future

* util.c (sprinttime): Increase buffer size.
* tests/stat64-v.test: Try to set modification time of the sample file
to a value set in the distant future.
diff --git a/util.c b/util.c
index b7476cf..570888c 100644
--- a/util.c
+++ b/util.c
@@ -425,7 +425,7 @@
 sprinttime(time_t t)
 {
 	struct tm *tmp;
-	static char buf[sizeof("yyyy/mm/dd-hh:mm:ss")];
+	static char buf[sizeof(int) * 3 * 6];
 
 	if (t == 0) {
 		strcpy(buf, "0");