Changing the frames map would lead to a crash later in undo.
This was because the `frames_storage` was not written to file corectly.
When the `frames_storage` was tagged dirty, it recreated the
`frames_storage.num` as well as the `keys` and `values` pointers.
The `keys` and `values` pointers were written after the update, but
since the `num` is just an `int` in the embeded struct, it was written
before being updated. This lead to an out-of-sync state and later
to the crash.
The fix makes sure we write the struct *after* updating it.
270a229728