fs: reject NaN durations instead of wrapping

strconv.ParseFloat accepts "NaN", and NaN compares false with every
bound, so it got past the overflow check added for 300y and was then
converted to a time.Duration, which gives the minimum int64 on amd64.
--max-age NaN or --min-age NaNM silently became about -292 years,
which moves the filter's ModTimeFrom into the future and excludes every
file, instead of being rejected.

fs/parseduration.go

@@ -61,6 +61,9 @@ func parseDurationSuffixes(age string) (time.Duration, error) {
 			if err != nil {
 				return time.Duration(0), err
 			}
+			if math.IsNaN(period) {
+				return time.Duration(0), fmt.Errorf("duration %q is not a number", age)
+			}
 			period *= float64(ageSuffix.Multiplier)
 			if math.Abs(period) >= math.MaxInt64 {
 				return time.Duration(0), fmt.Errorf("duration %q is too long", age)

fs/parseduration_test.go

@@ -58,6 +58,9 @@ func TestParseDuration(t *testing.T) {
 		{"300y", 0, true},
 		{"-300y", 0, true},
 		{"1e10d", 0, true},
+		{"NaN", 0, true},
+		{"NaNd", 0, true},
+		{"-NaNM", 0, true},
 		{"2001-02-03", now.Sub(time.Date(2001, 2, 3, 0, 0, 0, 0, time.Local)), false},
 		{"2001-02-03 10:11:12", now.Sub(time.Date(2001, 2, 3, 10, 11, 12, 0, time.Local)), false},
 		{"2001-08-03 10:11:12", now.Sub(time.Date(2001, 8, 3, 10, 11, 12, 0, time.Local)), false},