encoding/asn1: handle UTCTime before the year 2000

UTCTime only has a two digit date field and year values from 50 should
be 1950, not 2050.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/5729063

src/pkg/encoding/asn1/asn1.go

@@ -250,10 +250,14 @@ func parseBase128Int(bytes []byte, initOffset int) (ret, offset int, err error)
 func parseUTCTime(bytes []byte) (ret time.Time, err error) {
 	s := string(bytes)
 	ret, err = time.Parse("0601021504Z0700", s)
-	if err == nil {
-		return
-	}
+	if err != nil {
 		ret, err = time.Parse("060102150405Z0700", s)
+	}
+	if err == nil && ret.Year() >= 2050 {
+		// UTCTime only encodes times prior to 2050. See https://tools.ietf.org/html/rfc5280#section-4.1.2.5.1
+		ret = ret.AddDate(-100, 0, 0)
+	}
+
 	return
 }