message: [lldb][NFC] Assert on invalid cursors positions when creating CompletionRequest
  Before we just triggered undefined behavior on invalid positions.
diff: @@ -16,6 +16,7 @@ CompletionRequest::CompletionRequest(llvm::StringRef command_line,
  CompletionResult &result)
  : m_command(command_line), m_raw_cursor_pos(raw_cursor_pos),
  m_result(result) {
  + assert(raw_cursor_pos <= command_line.size() && "Out of bounds cursor?");
  // We parse the argument up to the cursor, so the last argument in
  // parsed_line is the one containing the cursor, and the cursor is after the
  