Fix how sullying of serialized objects is done.

There is an intermittent issue with unit test throwing a different error `BLST_POINT_NOT_ON_CURVE` vs `BLST_BAD_ENCODING` depending on the point that gets sullied. Is not a security issue, just an invalid test case. Need to update accordingly

Locate the unit test covering serialized objects and the code that sullies the test point. Reproduce the differing BLST_POINT_NOT_ON_CURVE and BLST_BAD_ENCODING errors, then determine the intended invalid-test behavior and update the test accordingly.

Written by the indexing model from the issue text.