clusterer_controller: guard the zero-length cipherstate passthrough

cl_ctr_send_join_req_pkt() sends Noise msg 1 with an empty authenticated
payload, passing payload = NULL, pl_len = 0. That reaches the no-key
passthrough in cl_ctr_cs_encrypt() as memcpy(ct, NULL, 0).

memcpy() declares both pointers nonnull, and passing NULL is undefined
even when the length is zero, so gcc-13 at -O9 inlines the whole chain
(send_join_req_pkt -> noise_write1 -> ss_encrypt_hash -> cs_encrypt),
proves the argument NULL and fails the build under -Werror=nonnull.

Skip the copy when there is nothing to copy, in both cs_encrypt() and
its cs_decrypt() counterpart, which has the same latent problem. No
behaviour change: the branch is only reachable before a key is mixed in,
and a zero-length copy moved no bytes anyway.

Author: Yury Kirsanov

modules/clusterer_controller/clusterer_controller.c

@@ -1926,7 +1926,9 @@ static int cl_ctr_cs_encrypt(cl_ctr_cipherstate_t *cs, const unsigned char *ad,
                          const unsigned char *pt, size_t pt_len, unsigned char *ct)
 {
     unsigned char nonce[12]; unsigned long long clen = 0;
-    if (!cs->has_key) { memcpy(ct, pt, pt_len); return (int)pt_len; }
+    /* pt may legitimately be NULL for an empty payload (Noise msg 1 sends
+     * one); memcpy() is nonnull even for a zero length, so guard it. */
+    if (!cs->has_key) { if (pt_len) memcpy(ct, pt, pt_len); return (int)pt_len; }
     cl_ctr_cs_nonce(cs->n, nonce);
     crypto_aead_chacha20poly1305_ietf_encrypt(ct, &clen, pt, pt_len,
                                               ad, ad_len, NULL, nonce, cs->k);
@@ -1937,7 +1939,7 @@ static int cl_ctr_cs_decrypt(cl_ctr_cipherstate_t *cs, const unsigned char *ad,
                          const unsigned char *ct, size_t ct_len, unsigned char *pt)
 {
     unsigned char nonce[12]; unsigned long long plen = 0;
-    if (!cs->has_key) { memcpy(pt, ct, ct_len); return (int)ct_len; }
+    if (!cs->has_key) { if (ct_len) memcpy(pt, ct, ct_len); return (int)ct_len; }
     cl_ctr_cs_nonce(cs->n, nonce);
     if (crypto_aead_chacha20poly1305_ietf_decrypt(pt, &plen, NULL, ct, ct_len,
                                                   ad, ad_len, nonce, cs->k) != 0)