fuzz: in FuzzedSock::GetSockName(), return a random-length name

ConsumeData() will always try to return a name as long as the requested size. It is more useful, and
closer to how `getsockname` would actually behave in reality, to return a random length name
instead.

This was hindering coverage in the PCP fuzz target as the addr len was set to the size of the
sockaddr_in struct and would exhaust all the provided data from the fuzzer.

Thanks to Marco Fleon for suggesting this.

Co-Authored-by: marcofleon <[email protected]>

Author: darosior

src/test/fuzz/util/net.cpp

@@ -358,8 +358,10 @@ int FuzzedSock::GetSockName(sockaddr* name, socklen_t* name_len) const
         return -1;
     }
     assert(name_len);
-    *name_len = m_fuzzed_data_provider.ConsumeData(name, *name_len);
-    if (*name_len < (int)sizeof(sockaddr)) return -1;
+    const auto bytes{ConsumeRandomLengthByteVector(m_fuzzed_data_provider, *name_len)};
+    if (bytes.size() < (int)sizeof(sockaddr)) return -1;
+    std::memcpy(name, bytes.data(), bytes.size());
+    *name_len = bytes.size();
     return 0;
 }