Fix small bug in RLE

Thanks @frrist

Signed-off-by: Jakub Sztandera <kubuxu@protocol.ai>

lib/rlepluslazy/bitvec.go

@@ -84,7 +84,7 @@ func (bv *wbitvec) Out() []byte {
 	}
 	if bv.bitCap > 8 {
 		// if we store some needed bits in second byte, save them also
-		bv.buf = append(bv.buf, byte(bv.bitCap>>8))
+		bv.buf = append(bv.buf, byte(bv.bits>>8))
 		bv.index++
 		bv.bits = bv.bits - 8
 	}

lib/rlepluslazy/runs.go

@@ -8,8 +8,8 @@ import (
 
 func Sum(a, b RunIterator) (RunIterator, error) {
 	it := addIt{a: a, b: b}
-	it.prep()
-	return &it, nil
+
+	return &it, it.prep()
 }
 
 type addIt struct {