LibVT: Don't crash when clicking outside of the terminal's buffer area

When resizing a terminal window the number of columns may change.
Previously we assumed that this also affects lines which were in the
terminal's buffer while that is not necessarily true.

Userland/Libraries/LibVT/TerminalWidget.cpp

@@ -592,8 +592,9 @@ VT::Position TerminalWidget::buffer_position_at(const Gfx::IntPoint& position) c
  column = 0;
  if (row >= m_terminal.rows())
  row = m_terminal.rows() - 1;
- if (column >= m_terminal.columns())
- column = m_terminal.columns() - 1;
+ auto& line = m_terminal.line(row);
+ if (column >= (int)line.length())
+ column = line.length() - 1;
  row += m_scrollbar->value();
  return { row, column };
 }
@@ -736,7 +737,7 @@ void TerminalWidget::doubleclick_event(GUI::MouseEvent& event)
  start_column = column;
  }
 
- for (int column = position.column(); column < m_terminal.columns() && (line.code_point(column) == ' ') == want_whitespace; ++column) {
+ for (int column = position.column(); column < (int)line.length() && (line.code_point(column) == ' ') == want_whitespace; ++column) {
  end_column = column;
  }