wire_sync_write() adds length, but we already have it, so use write_all.
sync_crypto_read() handed an on-stack buffer to cryptomsg_decrypt_header,
which expected a tal() pointer, so use the known length instead.
sync_crypto_read() also failed to read the tag; add that in (no
overflow possible as 16 is an int, len is a u16).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
It's awkward to handle them differently. But this change means we
need to expose them to the generated code.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
