1. tal_strndup(.., str, strlen(str)) == tal_strdup()
2. tal_strdup also takes(), so document that.
3. Avoid passing 'struct sha256' on the stack: use ptr.
4. Generally, structures shouldn't keep pointers to things they don't own.
In this case, mvt->node_id.
5. Make payment_hash a pointer, since NULL is more natural than an all-zero
hash.
And add NON_NULL_ARGS() to the functions; it's cumbersome, but make it
fairly clear what params are optional.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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