lib/sysfs: zero-terminate result of sysfs_blkdev_get_devchain()

sysfs_blkdev_get_devchain() does not zero-terminate its result, but the
caller expects a C-string.
Use ul_buffer_get_string() which includes a trailing zero-byte.

Now it's also valid for sz and bufsz to be equal.

See #3088.
Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>

lib/sysfs.c

@@ -404,8 +404,8 @@ char *sysfs_blkdev_get_devchain(struct path_cxt *pc, char *buf, size_t bufsz)
 	ul_buffer_append_string(&tmp, _PATH_SYS_DEVBLOCK "/");
 	ul_buffer_append_data(&tmp, buf, ssz);
 
-	p = ul_buffer_get_data(&tmp, &sz, NULL);
-	if (p && sz < bufsz) {
+	p = ul_buffer_get_string(&tmp, &sz, NULL);
+	if (p && sz <= bufsz) {
 		memcpy(buf, p, sz);
 		res = buf;
 	}