Clear decrypted block from stack (#1190)

src/libsodium/crypto_secretbox/crypto_secretbox_easy.c

@@ -134,6 +134,7 @@ crypto_secretbox_open_detached(unsigned char *m, const unsigned char *c,
     for (i = 0U; i < mlen0; i++) {
         m[i] = block0[crypto_secretbox_ZEROBYTES + i];
     }
+    sodium_memzero(block0, sizeof block0);
     if (clen > mlen0) {
         crypto_stream_salsa20_xor_ic(m + mlen0, c + mlen0, clen - mlen0,
                                      n + 16, 1U, subkey);