Ensure blob_attr length check does not perform out of bounds reads

Ensure blob_attr length check does not perform out of bounds reads

Before there might have been as little as one single byte left which
would result in 3 bytes of blob_attr->id_len being out of bounds.

Acked-by: Yousong Zhou <[email protected]>
Signed-off-by: Tobias Schramm <[email protected]>
[line wrapped < 72 chars]
Signed-off-by: Petr Štetiar <[email protected]>