Welcome: Fix reading of welcome.txt file

The buffer returned by read_line() used to be null-terminated, however
that was changed in 129a58a, resulting in some line strings containing
garbage data. Explicitly telling the String constructor the buffer's
size fixes that.

Fixes SerenityOS#4397.

Applications/Welcome/main.cpp

@@ -55,28 +55,17 @@ struct ContentPage {
 
 static Optional<Vector<ContentPage>> parse_welcome_file(const String& path)
 {
- const auto error = Optional<Vector<ContentPage>>();
  auto file = Core::File::construct(path);
-
  if (!file->open(Core::IODevice::ReadOnly))
- return error;
+ return {};
 
  Vector<ContentPage> pages;
  StringBuilder current_output_line;
  bool started = false;
  ContentPage current;
- while (true) {
+ while (file->can_read_line()) {
  auto buffer = file->read_line(4096);
- if (buffer.is_null()) {
- if (file->error()) {
- file->close();
- return error;
- }
-
- break;
- }
-
- auto line = String((char*)buffer.data());
+ auto line = String((const char*)buffer.data(), buffer.size());
  if (line.length() > 1)
  line = line.substring(0, line.length() - 1); // remove newline
  switch (line[0]) {
@@ -122,7 +111,6 @@ static Optional<Vector<ContentPage>> parse_welcome_file(const String& path)
  pages.append(current);
  }
 
- file->close();
  return pages;
 }