Do not send (potentially) invalid headers in response to getheaders
Do not send (potentially) invalid headers in response to getheaders

Nowhere else in the protocol do we send headers which are for
blocks we have not fully validated except in response to getheaders
messages with a null locator. On my public node I have not seen any
such request (whether for an invalid block or not) in at least two
years of debug.log output, indicating that this should have minimal
impact.

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