examples/fips_validation: fix crash on allocation failure

examples/fips_validation: fix crash on allocation failure

The return value was not being checked when calling the
get_writeback_data function in the AES test case. On failure, this led
to a NULL dereference when using memcpy later. The return value is now
checked to avoid this NULL dereference.

Coverity issue: 363463
Fixes: 952e10cdad5e ("examples/fips_validation: support scatter gather list")

Signed-off-by: Ciara Power <ciara.power@intel.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>