Strengthen MuSig nonce gen by feeding addresses of args

[jonasnick]

The musig nonce function looks as follows:

static void secp256k1_nonce_function_musig(secp256k1_scalar *k, const unsigned char *session_id, const unsigned char *key32, const unsigned char *msg32, const unsigned char *agg_pk, const unsigned char *extra_input32) {

There's a chance that if this nonce function is accidentally called twice with the same argument values, the argument pointers store different addresses. Thus, we could hash these locations like this:

secp256k1_sha256_write(&sha, (unsigned char *)&session_id, sizeof(session_id));

However, it should be possible to test the nonce function without this, to make sure that different values are sufficient to produce a different hash.

This idea was originally brought up by @elichai.

[apoelstra]

I like it. The address will maybe only have five or ten bits of entropy ... but to prevent lattice attacks in practice you pretty-much only need one (assuming your nonce, aside from being reused, is pretty high-entropy :))