[crypto] Clarify ASN.1 cursor invalidation behaviour

Calling asn1_skip_if_exists() on a malformed ASN.1 object may
currently leave the cursor in a partially-updated state, where the tag
byte and one of the length bytes have been stripped.  The cursor is
left with a valid data pointer and length and so no out-of-bounds
access can arise, but the cursor no longer points to the start of an
ASN.1 object.

Ensure that each ASN.1 cursor manipulation code path leads to the
cursor being either fully updated, left unmodified, or invalidated,
and update the function descriptions to reflect this.

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

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