Adds missing early returns

[Ruteri]

Adds missing early returns on unexpected measurements headers.
Thankfully this is not something that can be exploited, as:

• If a correct attestation is exchanged, the headers will correctly be overwritten
• If client attestation is expected but not present, as in builder-hub configuration, the request is dropped elsewhere (reproduced)
• the only problematic case is when no client attestation is expected, but then — the headers are not expected either

Will be adding follow-up regression tests.

[MoeMahhouk]

good catch