backends: add sanity check for the uploaded file size

Bugs in the error handling while uploading a file to the backend could
cause incomplete files, e.g. https://github.com/golang/go/issues/42400
which could affect the local backend.

Proactively add sanity checks which will treat an upload as failed if
the reported upload size does not match the actual file size.

internal/backend/local/local.go

@@ -118,11 +118,16 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 	}
 
 	// save data, then sync
-	_, err = io.Copy(f, rd)
+	wbytes, err := io.Copy(f, rd)
 	if err != nil {
 		_ = f.Close()
 		return errors.Wrap(err, "Write")
 	}
+	// sanity check
+	if wbytes != rd.Length() {
+		_ = f.Close()
+		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", wbytes, rd.Length())
+	}
 
 	if err = f.Sync(); err != nil {
 		pathErr, ok := err.(*os.PathError)