quick fix for Bitmap.Any bug

Want to make empty containers a thing of the past, but that can wait for another
day.

roaring/roaring.go

@@ -331,7 +331,7 @@ func (b *Bitmap) Any() bool {
 	// TODO (jaffee) I'm not sure if it's possible/legal to have an empty
 	// container, so this loop may be totally unnecessary. In theory, any empty
 	// container should be removed from the bitmap though.
-	for b := iter.Next(); b; iter.Next() {
+	for iter.Next() {
 		_, c := iter.Value()
 		if c.n > 0 {
 			return true

roaring/roaring_internal_test.go

@@ -3818,3 +3818,31 @@ func BenchmarkUnionInPlaceRegression(b *testing.B) {
 		}
 	})
 }
+
+func TestBitmapAny(t *testing.T) {
+	bm := NewBTreeBitmap()
+	if bm.Any() {
+		t.Error("empty bitmap should have Any()==false")
+	}
+	bm.Add(1)
+	if !bm.Any() {
+		t.Error("bitmap with 1 bit should have Any()==true")
+	}
+	bm.Add(100000)
+	if !bm.Any() {
+		t.Error("bitmap with 2 bits should have Any()==true")
+	}
+	bm.Remove(1)
+	if !bm.Any() {
+		t.Error("bitmap with 1 bit left after removing 1 should have Any()==true")
+	}
+	bm.Add(1)
+	bm = bm.Difference(NewBTreeBitmap(1))
+	if !bm.Any() {
+		t.Error("bitmap with 1 bit left after differencing 1 should have Any()==true")
+	}
+	bm.Remove(100000)
+	if bm.Any() {
+		t.Error("shouldn't be any left")
+	}
+}