keys: Guard against null match function in keyring_search_aux()

The "dead" key type has no match operation, and a search for keys of
this type can cause a null dereference in keyring_search_aux().
keyring_search() has a check for this, but request_keyring_and_link()
does not.  Move the check into keyring_search_aux(), covering both of
them.

This was fixed upstream by commit c06cfb08b88d ("KEYS: Remove
key_type::match in favour of overriding default by match_preparse"),
part of a series of large changes that are not suitable for
backporting.

CVE-2017-2647 / CVE-2017-6951

Reported-by: Igor Redko <redkoi@virtuozzo.com>
Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
References: https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2017-2647
Reported-by: idl3r <idler1984@gmail.com>
References: https://www.spinics.net/lists/keyrings/msg01845.html
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: David Howells <dhowells@redhat.com>

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