Proof-harness report shows abort() stub without coverage

There are some proof harness (e.g., `aws_cryptosdk_verify_header` and `aws_cryptosdk_sign_header`) that shows `abort()` stub on the report, but there is no indication that the entry point might ever reach it. Investigate why this happens and remove it to improve coverage results.

Start with the proof-harness reports for aws_cryptosdk_verify_header and aws_cryptosdk_sign_header, then trace why abort() appears as a stub without evidence that the entry point can reach it. Identify the proof-harness or reporting input responsible and confirm the corrected reports no longer show the spurious stub or reduced coverage.

Written by the indexing model from the issue text.