Fix infinite loop when parsing invalid DAP message.

This commit updates `ContentReader::read` to optionally close the
underlying `Reader` when a message is read that does not terminate
the DAP header immediately following the `Content-Length` header
field.

This fixes an infinite loop when parsing an invalid DAP message. In
particular, the infinite loop occurs when a cross-site HTTP request from
a browser running on the same host as a cppdap server is sent to the
server. As cross-site requests to the localhost can be triggered by
untrusted JavaScript, more validation of the DAP messages is required to
protect cppdap-based servers from malicious requests.

This commit introduces a new enum flag on both `Session` and
`ContentReader` to allow a user to indicate that the underlying `Reader`
should be closed when an invalid DAP message is received. The
server example has been updated to indicate that invalid messages
should result in closure of the underlying `Reader`.

include/dap/session.h

@@ -103,6 +103,14 @@ ResponseOrError<T>& ResponseOrError<T>::operator=(ResponseOrError&& other) {
 // Session
 ////////////////////////////////////////////////////////////////////////////////
 
+// An enum flag that controls how the Session handles invalid data.
+enum OnInvalidData {
+  // Ignore invalid data.
+  kIgnore,
+  // Close the underlying reader when invalid data is received.
+  kClose,
+};
+
 // Session implements a DAP client or server endpoint.
 // The general usage is as follows:
 // (1) Create a session with Session::create().
@@ -144,6 +152,9 @@ class Session {
   // create() constructs and returns a new Session.
   static std::unique_ptr<Session> create();
 
+  // Sets how the Session handles invalid data.
+  virtual void setOnInvalidData(OnInvalidData) = 0;
+
   // onError() registers a error handler that will be called whenever a protocol
   // error is encountered.
   // Only one error handler can be bound at any given time, and later calls