How to reinstall Safari 8 without reinstalling OS X Yosemite
First try to boot your computer into Safe Mode and open Safari. If it does open you can see if there are any extensions causing the crash and remove them.

You can also update to the Safari 8.0.1 beta from https://developer.apple.com/downloads/index.action# and see if that helps fix it.

If all else fails. Boot to the recovery partition and reinstall the OS (This will ONLY reinstall the core system and its Apps. Including Safari. No personal data will be lost)