common/buf/multi_buffer.go: Fix Compact() (#5015)

Fixes https://github.com/XTLS/Xray-core/issues/5012

Co-authored-by: patterniha <71074308+patterniha@users.noreply.github.com>

common/buf/multi_buffer.go

@@ -144,7 +144,7 @@ func Compact(mb MultiBuffer) MultiBuffer {
 
 	for i := 1; i < len(mb); i++ {
 		curr := mb[i]
-		if last.Len()+curr.Len() > Size {
+		if curr.Len() > last.Available() {
 			mb2 = append(mb2, last)
 			last = curr
 		} else {