Incorrect Assert in BufFileSize()?

I'm trying to figure out why BufFileSize() Asserts that file->fileset
isn't NULL, per 1a990b207.

The discussion for that commit is in [1], but I don't see any
explanation of the Assert in the discussion or commit message and
there's no comment explaining why it's there.

The code that comes after the Assert does not look at the fileset
field.  With the code as it is, it doesn't seem possible to get the
file size of a non-shared BufFile and I don't see any reason for that.

Should the Assert be checking file->files != NULL?

David

[1] https://postgr.es/m/CAH2-Wzn0ZNLZs3DhCYdLMv4xn1fnM8ugVHPvWz67dSUh1s_%3D2Q%40mail.gmail.com