`writedata` used to align the written buffer size to a multiple of 4. This
causes multiple issues:
* Writes uninitialized data.
* Crash with ASAN due to a heap buffer overflow if the buffer is not any longer
than what is passed in.
* Modifies the length of the buffer which can't be undone when reading the
buffer again.
I don't know of any reason for this alignment here. I'd think that it doesn't
matter when writing to a file. If it would matter, then we should probably align
to at least 8 nowadays because that's the alignment of pointers. The original
reason for this alignment seems to be lost to history. It was already part of
the initial commit.
Pull Request: https://projects.blender.org/blender/blender/pulls/129821
9d87291c40