time: prepare for 64-bit ints

The code was trying to interpret a uint32 n as a signed quantity
and then store it in an int. For this, int(n) currently works, but when
int becomes 64 bits one must write int(int32(n)) instead, to get
the 32-bit sign extension.

Update #2188.

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/6551068

src/pkg/time/zoneinfo_read.go

@@ -141,7 +141,7 @@ func loadZoneData(bytes []byte) (l *Location, err error) {
 		if n, ok = zonedata.big4(); !ok {
 			return nil, badData
 		}
-		zone[i].offset = int(n)
+		zone[i].offset = int(int32(n))
 		var b byte
 		if b, ok = zonedata.byte(); !ok {
 			return nil, badData