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_test.go

@@ -175,6 +175,29 @@ func TestCompact(t *testing.T) {
 	}
 }
 
+func TestCompactWithConsumed(t *testing.T) {
+	// make a consumed buffer (a.Start != 0)
+	a := New()
+	for range 8192 {
+		common.Must2(a.WriteString("a"))
+	}
+	a.Read(make([]byte, 2))
+
+	b := New()
+	for range 2 {
+		common.Must2(b.WriteString("b"))
+	}
+
+	mb := MultiBuffer{a, b}
+	cmb := Compact(mb)
+	mbc := &MultiBufferContainer{mb}
+	mbc.Read(make([]byte, 8190))
+
+	if w := cmb.String(); w != "bb" {
+		t.Error("unexpected Compact result ", w)
+	}
+}
+
 func BenchmarkSplitBytes(b *testing.B) {
 	var mb MultiBuffer
 	raw := make([]byte, Size)