test xmss_fast fails last iteration (#1023)

[matbok]

last iteration fails. Here is the output:

  - iteration #1022:
    smlen as expected [2532].
    verification succeeded.
    mlen as expected [32].
    output message as expected.
    flipping a bit of m invalidates signature.
  - iteration #1023:
    smlen as expected [2532].
  X verification failed!
  X mlen incorrect [0 != 32]!
  X output message incorrect!
    flipping a bit of m invalidates signature.

target machine: PC x86_64

[joostrijneveld]

This is a consequence of erroring on the side of caution when we run out of leafs, but this is admittedly a bit of an ugly outcome..