fix: avoid throwing an exception if the current selection no longer makes sense when initializing ySyncPlugin

[gvergnaud]

Fixes https://github.com/yjs/y-prosemirror/issues/170.

[dmonad]

Awesome PR!

• Precise bug report
• Reproducible code
• Working fix that even uses lib0.math instead of Math.

It took me just a few minutes and saved me a lot of time. Thanks!!

Will merge, and release now. There should be a new release with a fix in a few minutes.