Avoid usage of __CPROVER_havoc_object

[adpaco-aws]

A call to __CPROVER_havoc_object will overwrite an entire CBMC object. If the pointer being passed to write_unconstrained_data is part of a larger struct, CBMC will overwrite the larger struct: https://github.com/awslabs/aws-verification-model-for-libcrypto/blob/main/source/ec_override.c#L463-L470