Avoid use of uninitialised decode_error variable.

Pull initialisation of decode_error and invalid_key up to
tls_key_share_{client,server}_peer_public(), which are the entry points
for the key share code. The entry point was previously
tls_key_share_peer_public(), however with the introduction of MLKEM this
was split into separate client and server functions, without the
initialisation being included. Also initialise decode_error and
invalid_params on entry to tls_key_share_peer_params().

Code that reaches tls_key_share_client_peer_public_mlkem768x25519() could
previously result in code branching based on decode_error, which is
uninitialised stack based memory.

Thanks to Guido Vranken of Aisle Research for reporting this issue.

With and ok tb@

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