Selected Enemy/Boss Changes on Hardmode toggle

[krogenth]

Describe the bug
When toggling between hardmode/normal, the selected enemy/boss will sometimes be changed.

To Reproduce
Steps to reproduce the behavior:

1. Select an enemy/boss that isn't the first
2. Toggle the Hardmode checkbox a few times
3. Notice that the selected enemy/boss has changed

Expected behavior
It should try to stay with the same enemy index where possible, and only revert to the first when the selected index is out of the new bounds.