cmd: Don't read password from stdin for backup --stdin

cmd/restic/cmd_backup.go

@@ -262,6 +262,10 @@ func readFilenamesRaw(r io.Reader) (names []string, err error) {
 // Check returns an error when an invalid combination of options was set.
 func (opts BackupOptions) Check(gopts GlobalOptions, args []string) error {
 	if gopts.password == "" {
+		if opts.Stdin {
+			return errors.Fatal("cannot read both password and data from stdin")
+		}
+
 		filesFrom := append(append(opts.FilesFrom, opts.FilesFromVerbatim...), opts.FilesFromRaw...)
 		for _, filename := range filesFrom {
 			if filename == "-" {