file transfer: a send_confirm past the last file no longer panics

`set_stream_offset` indexed `self.files` directly. Its only guard is the
`self.file_num() == r.file_num` check in `confirm()`, and `file_num` counts up
past every file, so it equals `files.len()` once the job is done -- read_frame
at :849 treats exactly that value as "job done". A peer that then sends
`send_confirm` with the matching file_num and a non-zero OffsetBlk gets through
the equality check and off the end of the slice.

Every other site indexing `files` in this file already bounds-checks; this was
the one that did not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

libs/base/src/fs.rs

@@ -1106,6 +1106,9 @@ impl TransferJob {
     }
 
     async fn set_stream_offset(&mut self, file_num: usize, offset: u64) {
+        if file_num >= self.files.len() {
+            return;
+        }
         if let DataSource::FilePath(p) = &self.data_source {
             let entry = &self.files[file_num];
             let Some(path) = self.resolve_entry_path(p, &entry.name) else {