[crypto] Treat high tag numbers as invalid

ASN.1 allows for multi-byte tag numbers by setting the low five bits
of the first tag byte to 0x1f.  No tag that we need to handle has this
format, and the existing checks for specific tag numbers will already
fail to match against such a tag (treating it as a normal single-byte
tag number).

Refuse to parse any tag with a high tag number format, to guard
against future bugs that could arise because the tag length would be
calculated incorrectly.

Signed-off-by: Michael Brown <mcb30@ipxe.org>