Blender Git Loki

Fix T56108: Crash editing corrupted vertex groups

While the file in this report had corrupted values,
this is avoidable without adding any extra overhead.

Use unsigned vertex group indices since we don't need negative values,
this is an alternative to checking they aren't negative in many places.

Vertex group values over INT_MAX is still considered invalid,
so any accidental unsigned wrapping won't be silently ignored.