internal/restic: Fix panic in ParseDuration

Fixes #5485. Includes test case by @MichaelEischer.

internal/restic/duration.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"strconv"
 	"strings"
-	"unicode"
 
 	"github.com/restic/restic/internal/errors"
 )
@@ -52,7 +51,7 @@ func nextNumber(input string) (num int, rest string, err error) {
 	}
 
 	for i, s := range input {
-		if !unicode.IsNumber(s) {
+		if s < '0' || s > '9' {
 			rest = input[i:]
 			break
 		}

internal/restic/duration_test.go

@@ -37,6 +37,9 @@ func TestNextNumber(t *testing.T) {
 		{
 			input: "5d  ", num: 5, rest: "d  ",
 		},
+		{
+			input: "5", num: 5, rest: "",
+		},
 	}
 
 	for _, test := range tests {
@@ -78,6 +81,7 @@ func TestParseDuration(t *testing.T) {
 		{input: "2w", err: true},
 		{input: "1y4m3w1d", err: true},
 		{input: "s", err: true},
+		{input: "\xdf\x80", err: true}, // NKO DIGIT ZERO; we want ASCII digits
 	}
 
 	for _, test := range tests {