Fix cross reference table generator.

If the hash or public key algorithm is "undef" the signature type
will receive special handling and shouldn't be included in the
cross reference table.
Reviewed-by: Tim Hudson <tjh@openssl.org>

(cherry picked from commit 55f7fb8848b6e4bec291724a479e1580d6f407d6)

Conflicts:
	crypto/objects/obj_xref.h