Add tests for ReadAt of ByteView

The tests pass if ReadAt returns in errors for invalid values of offset.
The patch also adds check for error to be returned when dest is larger
in size than source.

byteview_test.go

@@ -139,6 +139,23 @@ func TestByteViewSlice(t *testing.T) {
 	}
 }
 
+func TestByteViewReadAt(t *testing.T) {
+	tests := []int64{
+		-1,
+		5,
+	}
+	bv := ByteView{b: []byte("xy")}
+	for i, tt := range tests {
+		if _, re := bv.ReadAt([]byte("ab"), tt); re == nil {
+			name := fmt.Sprintf("test %d", i)
+			t.Errorf("got nil; want error, for offset %d", name, tt)
+		}
+	}
+	if _, re := bv.ReadAt([]byte("abcd"), 0); re == nil {
+		t.Errorf("got nil; want error, when dest is larger")
+	}
+}
+
 func min(a, b int) int {
 	if a < b {
 		return a