Clamp initial indexes before measuring covers

[vanderlee]

What changed

• Return cleanly when a coverflow has no child covers.
• Clamp the stored option index, current index, and requested index before the first frame is measured.
• Preserve the file's existing line-ending format.

Why

When an initial index is larger than the number of covers, _setIndex() clamps only its local argument but calls _frame() while options.index is still out of range. _frame() then dereferences an absent cover's offsetWidth.

This directly addresses the failure reported with fewer than eight images and an initial index near the end of a larger collection.

Closes #45.