fuzz: simplify FuzzedSock::m_peek_data

`FuzzedSock::m_peek_data` need not be an optional of a vector.
It can be just a vector whereas an empty vector denotes "no peek data".

src/test/fuzz/util/net.cpp

@@ -191,11 +191,11 @@ ssize_t FuzzedSock::Recv(void* buf, size_t len, int flags) const
     }
     std::vector<uint8_t> random_bytes;
     bool pad_to_len_bytes{m_fuzzed_data_provider.ConsumeBool()};
-    if (m_peek_data.has_value()) {
+    if (!m_peek_data.empty()) {
         // `MSG_PEEK` was used in the preceding `Recv()` call, return `m_peek_data`.
-        random_bytes = m_peek_data.value();
+        random_bytes = m_peek_data;
         if ((flags & MSG_PEEK) == 0) {
-            m_peek_data.reset();
+            m_peek_data.clear();
         }
         pad_to_len_bytes = false;
     } else if ((flags & MSG_PEEK) != 0) {

src/test/fuzz/util/net.h

@@ -43,7 +43,7 @@ class FuzzedSock : public Sock
      * If `MSG_PEEK` is used, then our `Recv()` returns some random data as usual, but on the next
      * `Recv()` call we must return the same data, thus we remember it here.
      */
-    mutable std::optional<std::vector<uint8_t>> m_peek_data;
+    mutable std::vector<uint8_t> m_peek_data;
 
     /**
      * Whether to pretend that the socket is select(2)-able. This is randomly set in the