ANDROID: dm: android-verity: fix table_make_digest() error handling

If table_make_digest() fails, verify_verity_signature() would try to
pass the returned ERR_PTR() to kfree().

This fixes the smatch error:

drivers/md/dm-android-verity.c:601 verify_verity_signature() error: 'pks' dereferencing possible ERR_PTR()

Change-Id: I9b9b7764b538cb4a5f94337660e9b0f149b139be
Signed-off-by: Greg Hackmann <ghackmann@google.com>