s390/crypto: xts-aes-s390 fix extra run-time crypto self tests finding

With 'extra run-time crypto self tests' enabled, the selftest
for s390-xts fails with

  alg: skcipher: xts-aes-s390 encryption unexpectedly succeeded on
  test vector "random: len=0 klen=64"; expected_error=-22,
  cfg="random: inplace use_digest nosimd src_divs=[2.61%@+4006,
  84.44%@+21, 1.55%@+13, 4.50%@+344, 4.26%@+21, 2.64%@+27]"

This special case with nbytes=0 is not handled correctly and this
fix now makes sure that -EINVAL is returned when there is en/decrypt
called with 0 bytes to en/decrypt.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>