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>

scan-tree.c

@@ -42,6 +42,7 @@ char *readfile(const char *path)
 
 	if (!(f = fopen(path, "r")))
 		return NULL;
+	buf[0] = 0;
 	fgets(buf, MAX_PATH, f);
 	fclose(f);
 	return buf;

ui-repolist.c

@@ -23,6 +23,7 @@ time_t read_agefile(char *path)
 
 	if (!(f = fopen(path, "r")))
 		return -1;
+	buf[0] = 0;
 	if (fgets(buf, sizeof(buf), f) == NULL)
 		return -1;
 	fclose(f);