Reject invalid varints when decoding.

Fixes #7.
Fixes #11.

decode.go

@@ -27,7 +27,7 @@ func DecodedLen(src []byte) (int, error) {
 // that the length header occupied.
 func decodedLen(src []byte) (blockLen, headerLen int, err error) {
 	v, n := binary.Uvarint(src)
-	if n == 0 {
+	if n <= 0 {
 		return 0, 0, ErrCorrupt
 	}
 	if uint64(int(v)) != v {

snappy_test.go

@@ -78,6 +78,16 @@ func TestSmallRegular(t *testing.T) {
 	}
 }
 
+func TestInvalidVarint(t *testing.T) {
+	data := []byte("\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00")
+	if _, err := DecodedLen(data); err != ErrCorrupt {
+		t.Errorf("DecodedLen: got %v, want ErrCorrupt", err)
+	}
+	if _, err := Decode(nil, data); err != ErrCorrupt {
+		t.Errorf("Decode: got %v, want ErrCorrupt", err)
+	}
+}
+
 func cmp(a, b []byte) error {
 	if len(a) != len(b) {
 		return fmt.Errorf("got %d bytes, want %d", len(a), len(b))