VimEditingEngine: Prevent crash upon 'x' or 'v' on blank lines

Previously, pressing 'x' for deletion on an otherwise empty line
insinuated a crash in TextEditor because a nonexistent code point was
accessed -- likewise for visual mode.

Userland/Libraries/LibGUI/TextDocument.cpp

@@ -350,7 +350,14 @@ String TextDocument::text_in_range(const TextRange& a_range) const
  auto& line = this->line(i);
  size_t selection_start_column_on_line = range.start().line() == i ? range.start().column() : 0;
  size_t selection_end_column_on_line = range.end().line() == i ? range.end().column() : line.length();
- builder.append(Utf32View(line.code_points() + selection_start_column_on_line, selection_end_column_on_line - selection_start_column_on_line));
+
+ if (!line.is_empty()) {
+ builder.append(
+ Utf32View(
+ line.code_points() + selection_start_column_on_line,
+ selection_end_column_on_line - selection_start_column_on_line));
+ }
+
  if (i != range.end().line())
  builder.append('\n');
  }
@@ -925,12 +932,16 @@ void TextDocument::remove(const TextRange& unnormalized_range)
  } else {
  // Delete across a newline, merging lines.
  VERIFY(range.start().line() == range.end().line() - 1);
+
  auto& first_line = line(range.start().line());
  auto& second_line = line(range.end().line());
+
  Vector<u32> code_points;
  code_points.append(first_line.code_points(), range.start().column());
- code_points.append(second_line.code_points() + range.end().column(), second_line.length() - range.end().column());
+ if (!second_line.is_empty())
+ code_points.append(second_line.code_points() + range.end().column(), second_line.length() - range.end().column());
  first_line.set_text(*this, move(code_points));
+
  remove_line(range.end().line());
  }