prevent panic in Bitmap.UnmarshalBinary when there is no data

api.go

@@ -268,6 +268,10 @@ func setUpImportOptions(opts ...ImportOption) (*ImportOptions, error) {
 // of the rows in this shard of this field concatenated together in one long
 // bitmap.
 func (api *API) ImportRoaring(ctx context.Context, indexName, fieldName string, shard uint64, remote bool, data []byte, opts ...ImportOption) (err error) {
+	if len(data) == 0 {
+		return errors.New("no data to import")
+	}
+
 	if err = api.validate(apiField); err != nil {
 		return errors.Wrap(err, "validating api method")
 	}

fragment.go

@@ -25,6 +25,7 @@ import (
 	"hash"
 	"io"
 	"io/ioutil"
+	"math"
 	"os"
 	"sort"
 	"sync"
@@ -33,9 +34,6 @@ import (
 	"unsafe"
 
 	"github.com/cespare/xxhash"
-
-	"math"
-
 	"github.com/gogo/protobuf/proto"
 	"github.com/pilosa/pilosa/internal"
 	"github.com/pilosa/pilosa/pql"

roaring/roaring.go

@@ -3465,6 +3465,10 @@ func readOfficialHeader(buf []byte) (size uint32, containerTyper func(index uint
 // UnmarshalBinary decodes b from a binary-encoded byte slice. data can be in
 // either official roaring format or Pilosa's roaring format.
 func (b *Bitmap) UnmarshalBinary(data []byte) error {
+	if data == nil {
+		// Nothing to unmarshal
+		return nil
+	}
 	fileMagic := uint32(binary.LittleEndian.Uint16(data[0:2]))
 	if fileMagic == magicNumber { // if pilosa roaring
 		return errors.Wrap(b.unmarshalPilosaRoaring(data), "unmarshaling as pilosa roaring")