ui-repolist: initialize char *buf to NULL

readfile() can fail if the agefile is not readable. Make sure free()
does not free an ininitialized string.

Signed-off-by: Christian Hesse <mail@eworm.de>

ui-repolist.c

@@ -15,7 +15,7 @@ static time_t read_agefile(char *path)
 {
 	time_t result;
 	size_t size;
-	char *buf;
+	char *buf = NULL;
 	struct strbuf date_buf = STRBUF_INIT;
 
 	if (readfile(path, &buf, &size)) {