QUIC sniffer: Optimize the code (#4655)

Based on https://github.com/XTLS/Xray-core/commit/2eed70e17dda9db003ecf6aa7ca5ce88f76f7a38

app/dispatcher/default.go

@@ -43,7 +43,7 @@ func (r *cachedReader) Cache(b *buf.Buffer, deadline time.Duration) error {
 		r.cache, _ = buf.MergeMulti(r.cache, mb)
 	}
 	b.Clear()
-	rawBytes := b.Extend(b.Cap())
+	rawBytes := b.Extend(min(r.cache.Len(), b.Cap()))
 	n := r.cache.Copy(rawBytes)
 	b.Resize(0, int32(n))
 	r.Unlock()