Add ARG_CHECKs to ensure "array of pointers" elements are non-NULL

[theStack]

We currently have five public API functions that take an "array of pointers" as input parameter:

• secp256k1_ec_pubkey_combine (ins: array of pointers to public keys to add)
• secp256k1_ec_pubkey_sort (pubkeys: array of pointers to public keys to sort)
• secp256k1_musig_pubkey_agg (pubkeys: array of pointers to public keys to aggregate)
• secp256k1_musig_nonce_agg (pubnonces: array of pointers to public nonces to aggregate)
• secp256k1_musig_partial_sig_agg (partial_sigs: array of pointers to partial signatures to aggregate)

Out of these, only _ec_pubkey_combine verifies that the individual pointer elements in the array are non-NULL each:

secp256k1/src/secp256k1.c

Lines 774 to 775 in e7f7083

	for (i = 0; i < n; i++) {
	ARG_CHECK(pubnonces[i] != NULL);

This PR adds corresponding ARG_CHECKS for the other API functions as well, in order to avoid running into potential UB due to NULL pointer dereference. It seems to me that the tiny run-time overhead is worth it doing this for consistency and to help users in case the arrays are set up incorrectly (I'm thinking e.g. of language binding writers where getting this right might be a bit more involved).

Looking into this was motivated by a review of furszy (thanks!), who pointed out that the non-NULL checks are missing in at least one API function in the silentpayments module PR as well. Happy to add some CHECK_ILLEGAL tests if there is conceptual support for this PR.

[furszy]

Cool, the review paid off so fast :). Would be good to add test coverage for it.

[kevkevinpal]

Does it make sense to check n_pubkeys is greater than 0?

Suggested change

	for (i = 0; i < n_pubkeys; i++) {
	ARG_CHECK(n_pubkeys > 0);
	for (i = 0; i < n_pubkeys; i++) {