The head node in the keyblock list is a PKT_NONE packet in GnuPG 1.4.2 and
later, and build_packet() aborts if you pass it one of those. If we iterate
through the packet list with walk_kbnode(), we'll automatically skip over
the problematic node, so do that instead.

See T515: gnupg 1.4.2 aborts() when generating key pairs in batch mode