Deal gracefully with a NULL or empty scatterlist which can happen
if both cryptlen and assoclen are zero and we're doing in-place
AEAD encryption.

This fixes a crash when this causes us to try and map a NULL page,
at least with some platforms / DMA mapping configs.

Cc: stable@vger.kernel.org # v4.19+
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>