Fixed ShatterSentinel magic spell

[Evyatar108]

Treating two bugs

1. A case where the the sentinel is destroyed inside the loop but isn't deleted from the summoned sentinels list
2. Removing a sentinel from the list while still iterating it will cause the list to shrink while still having the same index 'i' which causes you to miss the next sentinel.

These two bugs are very common and spread throughout this mod, I'm adding a fix here only as a FYI