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:

aws-encryption-sdk-c/verification/cbmc/sources/openssl/ec_override.c

Lines 486 to 492 in c51bf18

	void write_unconstrained_data(unsigned char *out, size_t len) {
	assert(AWS_MEM_IS_WRITABLE(out, len));
	// Currently we ignore the len parameter and just fill the entire buffer with unconstrained data.
	// This is fine because it is strictly more general behavior than writing only len bytes.
	__CPROVER_havoc_object(out);
	}

Issues related: #652