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 <yszhou4tech@gmail.com>
Signed-off-by: Tobias Schramm <tobleminer@gmail.com>
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Signed-off-by: Petr Štetiar <ynezz@true.cz>