cryptonote_basic: fix add_extra_nonce_to_tx_extra() length

Missing unit tests.

src/common/varint.h

@@ -125,4 +125,14 @@ namespace tools {
     int read_varint(InputIt &&first, InputIt &&last, T &i) {
     return read_varint<std::numeric_limits<T>::digits>(std::forward<InputIt>(first), std::forward<InputIt>(last), i);
   }
+
+  template <typename T, typename = std::enable_if_t<std::is_integral<T>::value && std::is_unsigned<T>::value>>
+  constexpr std::size_t get_varint_byte_size(T val) {
+    std::size_t bytes = 0;
+    do {
+      ++bytes;
+      val >>= 7;
+    } while (val);
+    return bytes;
+  }
 }

src/cryptonote_basic/cryptonote_format_utils.cpp

@@ -741,15 +741,19 @@ namespace cryptonote
   {
     CHECK_AND_ASSERT_MES(extra_nonce.size() <= TX_EXTRA_NONCE_MAX_COUNT, false, "extra nonce could be 255 bytes max");
     size_t start_pos = tx_extra.size();
-    tx_extra.resize(tx_extra.size() + 2 + extra_nonce.size());
+    const std::size_t len_varint_bytes = tools::get_varint_byte_size(extra_nonce.size());
+    tx_extra.resize(tx_extra.size() + 1 + len_varint_bytes + extra_nonce.size());
     //write tag
     tx_extra[start_pos] = TX_EXTRA_NONCE;
     //write len
     ++start_pos;
-    tx_extra[start_pos] = static_cast<uint8_t>(extra_nonce.size());
+    unsigned char * vp = tx_extra.data() + start_pos;
+    tools::write_varint(vp, extra_nonce.size());
+    assert(vp == tx_extra.data() + tx_extra.size() - extra_nonce.size());
     //write data
-    ++start_pos;
+    start_pos += len_varint_bytes;
-    memcpy(&tx_extra[start_pos], extra_nonce.data(), extra_nonce.size());
+    if (!extra_nonce.empty())
+      memcpy(&tx_extra[start_pos], extra_nonce.data(), extra_nonce.size());
     return true;
   }
   //---------------------------------------------------------------