We are using a single member in struct file_handle, the "name"

field, for more than one purpose.  When it begins with '"', it's a
file name; otherwise, it's a token that can be used to identify
it.  When that assertion fires, it's because we searched for the
name case-sensitively as a file name (so that there was no match),
and then we try to insert it case-insensitively as a token, which
fails because duplicates aren't allowed.

Solution: break the two purposes into two separate fields.  This
fixes the problem and likely makes the code easier to read too.

Fixes bug #18922.  Thanks to John Darrington for bug report and
review.