Fix crash when deleting image while selection tool is active

[Slashxdd]

Fixes #92

Root cause

NullReferenceException in GetSelectionRect() at originalImage.Width.

The exact crash chain:

1. Selection tool is open (selForm != null)
2. User presses Delete — deleteButton_Click fires (Delete is bound as ShortcutKeys on deleteBtn)
3. deleteButton_Click disposes the image and sets originalImage = null
4. Then calls NextFile() → OpenFile() → openImage()
5. Inside openImage(), selectionBtn.Checked = false fires selectionBtn_CheckedChanged → selForm.Dispose()
6. During disposal, WinForms fires the SizeChanged event on selForm
7. SelForm_SizeChanged calls GetSelectionRect()
8. GetSelectionRect() accesses originalImage.Width → originalImage is null → crash

Fix

One-line null guard at the entry of SelForm_SizeChanged:

private void SelForm_SizeChanged(object sender, EventArgs e)
{
    if (originalImage == null) return;  // ← added
    Rectangle r = GetSelectionRect();
    ...
}

GetSelectionRect() is only meaningful when an image is loaded, so returning early when originalImage is null is always correct.