Do not use a zero nonce in randombytes_buf_deterministic()

This can mitigate implications of reusing the same key across different
functions.

src/libsodium/randombytes/randombytes.c

@@ -165,7 +165,9 @@ void
 randombytes_buf_deterministic(void * const buf, const size_t size,
                               const unsigned char seed[randombytes_SEEDBYTES])
 {
-    static const unsigned char zero[crypto_stream_chacha20_ietf_NONCEBYTES] = { 0 };
+    static const unsigned char nonce[crypto_stream_chacha20_ietf_NONCEBYTES] = {
+        'L', 'i', 'b', 's', 'o', 'd', 'i', 'u', 'm', 'D', 'R', 'G'
+    };
 
     COMPILER_ASSERT(randombytes_SEEDBYTES == crypto_stream_chacha20_ietf_KEYBYTES);
 #if SIZE_MAX > 0x4000000000ULL
@@ -174,7 +176,7 @@ randombytes_buf_deterministic(void * const buf, const size_t size,
     }
 #endif
     crypto_stream_chacha20_ietf((unsigned char *) buf, (unsigned long long) size,
-                                zero, seed);
+                                nonce, seed);
 }
 
 size_t

test/default/randombytes.exp

@@ -1,2 +1,2 @@
-39fd2b7dd9c5196a8dbd0377b8dc4a498a35d86fbcde6accb2cc7d4cd8ea24922b23cce7a26023ab3f0eef693ac87f64258235eab1f7a32dc22762a0485b410c18b84231ade6a6d113615c61af434e27f8b1f3f5e1ad5b5cecf8fc122a35755c7208086d (deterministic)
+0d8e6cc68715648926732e7ea73250cfaf2d58422083904c841a8ba33b986111f346ba50723a68ae283524a6bded09f83be6b80595856f72e25b86918e8b114bafb94bc8abedd73daab454576b7c5833eb0bf982a1bb4587a5c970ff0810ca3b791d7e12 (deterministic)
 OK