time: reject tzdata with no zones

Fixes #29437

Change-Id: Ice0a03a543e564d66651bfdfce5cd32ebaa35926
Reviewed-on: https://go-review.googlesource.com/c/155746
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/zoneinfo_read.go

@@ -216,7 +216,13 @@ func LoadLocationFromTZData(name string, data []byte) (*Location, error) {
 	// Now we can build up a useful data structure.
 	// First the zone information.
 	//	utcoff[4] isdst[1] nameindex[1]
-	zone := make([]zone, n[NZone])
+	nzone := n[NZone]
+	if nzone == 0 {
+		// Reject tzdata files with no zones. There's nothing useful in them.
+		// This also avoids a panic later when we add and then use a fake transition (golang.org/issue/29437).
+		return nil, badData
+	}
+	zone := make([]zone, nzone)
 	for i := range zone {
 		var ok bool
 		var n uint32

src/time/zoneinfo_test.go

@@ -173,3 +173,12 @@ func TestEarlyLocation(t *testing.T) {
 		t.Errorf("Zone offset == %d, want %d", tzOffset, want)
 	}
 }
+
+func TestMalformedTZData(t *testing.T) {
+	// The goal here is just that malformed tzdata results in an error, not a panic.
+	issue29437 := "TZif\x00000000000000000\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0000"
+	_, err := time.LoadLocationFromTZData("abc", []byte(issue29437))
+	if err == nil {
+		t.Error("expected error, got none")
+	}
+}