Test chacha20 with length == 0

src/libsodium/crypto_stream/chacha20/ref/stream_chacha20_ref.c

@@ -101,7 +101,7 @@ chacha_encrypt_bytes(chacha_ctx *x, const u8 *m, u8 *c, unsigned long long bytes
     unsigned long long i;
 
     if (!bytes) {
-        return;
+        return; /* LCOV_EXCL_LINE */
     }
     j0 = x->input[0];
     j1 = x->input[1];

test/default/chacha20.c

@@ -35,6 +35,10 @@ static void tv(void)
     } while (++i < (sizeof tests) / (sizeof tests[0]));
 
     memset(out, 0x42, sizeof out);
+
+    assert(crypto_stream_chacha20_xor(out, out, 0U, nonce, key) == 0);
+    assert(crypto_stream_chacha20_xor_ic(out, out, 0U, nonce, 1U, key) == 0);
+
     crypto_stream_chacha20_xor(out, out, sizeof out, nonce, key);
     sodium_bin2hex(out_hex, sizeof out_hex, out, sizeof out);
     printf("[%s]\n", out_hex);