Fix length checks in cert_load()

Fix length checks in cert_load()

cert_load() iterates over multiple blobs, so the length argument to
blob_parse_untrusted() needs to be updated to prevent out-of-bounds
accesses.

Some other checks have become redundant and are removed, as
blob_parse_untrusted() already ensures that all attrs are contained in
the passed buffer.

Note that this issue currently does not pose a security threat, as an
over-restrictive check in blob_parse_untrusted() broke parsing of
buffers with multiple blobs completely.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>