fix use nil parameter, the MinMaxBig func will throw panic's problem

Author: ChicChip

roaring64/bsi64.go

@@ -509,6 +509,10 @@ func (b *BSI) MinMaxBig(parallelism int, op Operation, foundSet *Bitmap) *big.In
 
 	resultsChan := make(chan *big.Int, n)
 
+	if foundSet == nil {
+		foundSet = &b.eBM
+	}
+
 	card := foundSet.GetCardinality()
 	x := card / uint64(n)
 

roaring64/bsi64_test.go

@@ -649,6 +649,12 @@ func TestMinMaxWithRandom(t *testing.T) {
 	assert.Equal(t, max, bsi.MinMax(0, MAX, bsi.GetExistenceBitmap()))
 }
 
+func TestMinMaxWithNilFoundSet(t *testing.T) {
+	bsi, min, max := setupRandom()
+	assert.Equal(t, min, bsi.MinMax(0, MIN, nil))
+	assert.Equal(t, max, bsi.MinMax(0, MAX, nil))
+}
+
 func TestBSIWriteToReadFrom(t *testing.T) {
 	file, err := ioutil.TempFile("./testdata", "bsi-test")
 	if err != nil {