[MC-236284] Withers with large Invul turn upside down

This has been an issue ever since withers were added.

The Invul tag is not capped, and neither is the size decreased based on that tag. When a wither has an Invul value over 870, it starts to get a negative size and turn upside down.

Steps to reproduce:

Use the command
```
/summon wither ~ ~ ~ {Invul:2000}
```
Observe an upside-down wither that shrinks over time, until it becomes size 0, and then grows again.

Expected behavior:

The wither does not turn upside down (either by having a minimum size, or by capping of the NBT at 860 (before it reaches a size of 0)).