Previously, opening a zip with (*Reader).Open could result in a panic if
the zip contained a file whose name was exclusively made up of slash
characters or ".." path elements.
Open could also panic if passed the empty string directly as an argument.
Now, any files in the zip whose name could not be made valid for
fs.FS.Open will be skipped, and no longer added to the fs.FS file list,
although they are still accessible through (*Reader).File.
Note that it was already the case that a file could be accessible from
(*Reader).Open with a name different from the one in (*Reader).File, as
the former is the cleaned name, while the latter is the original one.
Finally, made the actual panic site robust as a defense-in-depth measure.