truncate buffer before reading empty files

If readfile() reads an empty file, fgets() won't truncate the buffer
and it'll still contain the contents of the previously read file.

[lh: fixed similar issue in ui-repolist.c]

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/ui-repolist.c b/ui-repolist.c
index 2c13d50..3aedde5 100644
--- a/ui-repolist.c
+++ b/ui-repolist.c
@@ -23,6 +23,7 @@
 
 	if (!(f = fopen(path, "r")))
 		return -1;
+	buf[0] = 0;
 	if (fgets(buf, sizeof(buf), f) == NULL)
 		return -1;
 	fclose(f);