The memfile undo data-block change detection didn't work for meshes
because we ended up writing a new pointer every time. In practice the
array the pointer references is always empty anyway, so we can just add
a check and write null instead.
Unfortunately this fix only applies to 4.5, since the attribute DNA
data (which is actually used at runtime in 5.0) is created temporarily
specifically for writing, so it gets a new address every time.
We'll probably need to solve #127706 in 5.0 to fix this.
Pull Request: https://projects.blender.org/blender/blender/pulls/141457
0ce5738415