object: Avoid crashing on zero-length AllowedMechanisms attribute

As returned by SoftHSM: softhsm/SoftHSMv2#825

Fixes: #323

Signed-off-by: Jakub Jelen <[email protected]>

Author: Jakuje

cryptoki/src/object.rs

@@ -1114,6 +1114,9 @@ impl TryFrom<CK_ATTRIBUTE> for Attribute {
                 Ok(Attribute::ValidationVersion(Version::new(val[0], val[1])))
             }
             AttributeType::AllowedMechanisms => {
+                if attribute.ulValueLen == 0 {
+                    return Ok(Attribute::AllowedMechanisms(Vec::<MechanismType>::new()));
+                }
                 let val = unsafe {
                     std::slice::from_raw_parts(
                         attribute.pValue as *const CK_MECHANISM_TYPE,