OSS-Fuzz: Add new fuzzer/options targets websocket (#2663)

Add websocket_fuzzer covering frame reading, UTF-8 and close payload validation, and the upgrade response parser. Add websocket_fuzzer.options to raise max_len above the 64 KiB frame length boundary.

test/fuzzing/websocket_fuzzer.cc

@@ -0,0 +1,54 @@
+#include <cstdint>
+#include <string>
+
+#include <httplib.h>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  if (size < 2) { return 0; }
+
+  auto selector = data[0];
+  std::string input(reinterpret_cast<const char *>(data + 1), size - 1);
+
+  switch (selector % 4) {
+  case 0: {
+    // read_websocket_frame. WebSocket::read passes the macro, close() passes
+    // 125, and 0 means no limit
+    static const size_t max_lens[] = {0, 125,
+                                      CPPHTTPLIB_WEBSOCKET_MAX_PAYLOAD_LENGTH};
+    auto expect_masked = (selector / 4) % 2 != 0;
+    auto max_len = max_lens[(selector / 8) % 3];
+    httplib::detail::BufferStream strm;
+    strm.write(input.data(), input.size());
+    auto opcode = httplib::ws::Opcode::Text;
+    std::string payload;
+    auto fin = false;
+    httplib::ws::impl::read_websocket_frame(strm, opcode, payload, fin,
+                                            expect_masked, max_len);
+    break;
+  }
+  case 1: {
+    // is_valid_utf8
+    httplib::ws::impl::is_valid_utf8(input);
+    break;
+  }
+  case 2: {
+    // is_valid_close_payload
+    httplib::ws::impl::is_valid_close_payload(input);
+    break;
+  }
+  case 3: {
+    // read_websocket_upgrade_response, with a fixed key since
+    // perform_websocket_handshake draws a random one
+    static const auto expected_accept =
+        httplib::detail::websocket_accept_key("dGhlIHNhbXBsZSBub25jZQ==");
+    httplib::detail::BufferStream strm;
+    strm.write(input.data(), input.size());
+    httplib::detail::WebSocketUpgradeResponse upgrade;
+    httplib::detail::read_websocket_upgrade_response(strm, expected_accept,
+                                                     "chat, stream", upgrade);
+    break;
+  }
+  }
+
+  return 0;
+}