examples/fips_validation: fix null dereferences

examples/fips_validation: fix null dereferences

One issue caught by Coverity 343408
*deref_parm: Directly dereferencing parameter val->val.

In writeback_tdes_hex_str(), tmp_val is initialised to null.
tmp_val.val is updated only if keys are found.
If keys are not found,it doesn't fails but continues
to invoke writeback_hex_str(),where val->val is accessed
without null check.

The fix is to return the error,
if keys are not found in writeback_tdes_hex_str().

Coverity issue: 343408
Fixes: 527cbf3d5e ("examples/fips_validation: support TDES parsing")
Cc: stable@dpdk.org
Signed-off-by: Chaitanya Babu Talluri <tallurix.chaitanya.babu@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>