crypto: caam - handle zero-length AEAD output

Recent AEAD changes in testmgr framework introduced by commit
a0d608ee

 ("crypto: testmgr - unify the AEAD encryption and decryption test vectors")
uncovered an error in the CAAM drivers, since they don't correctly
handle the case when AEAD output length is zero.

Add checks to avoid feeding zero-length req->dst to DMA API.

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>