[C++] use m_actingBlockLength in decodeLength() (#1045) #1211

Job	Run time
C++ Clang 9 (Ubuntu 20.04)	1m 24s
C++ Clang 10 (Ubuntu 20.04)	1m 52s
C++ Clang 13 (Ubuntu 24.04)	1m 30s
C++ Clang 11 (Ubuntu 20.04)	1m 30s
C++ Xcode (macOS)	57s
C++ Clang 14 (Ubuntu 24.04)	1m 26s
C++ GCC 6 (Ubuntu 20.04)	1m 26s
C++ Clang 12 (Ubuntu 20.04)	1m 31s
C++ Clang 15 (Ubuntu 24.04)	1m 26s
C++ GCC 7 (Ubuntu 20.04)	1m 25s
C++ Clang 16 (Ubuntu 24.04)	1m 20s
C++ GCC 11 (Ubuntu 24.04)	1m 33s
C++ GCC 8 (Ubuntu 20.04)	1m 25s
C++ Clang 17 (Ubuntu 24.04)	1m 27s
C++ GCC 12 (Ubuntu 24.04)	1m 31s
C++ GCC 9 (Ubuntu 20.04)	1m 30s
C++ MSVC (Windows) (windows-latest)	4m 35s
C++ Clang 18 (Ubuntu 24.04)	1m 33s
C++ GCC 13 (Ubuntu 24.04)	1m 17s
C++ GCC 10 (Ubuntu 20.04)	1m 21s
C++ GCC 14 (Ubuntu 24.04)	1m 24s
C# 8.0.x	2m 38s
Golang 1.22.x	2m 4s
Java 17 (ubuntu-24.04)	1m 35s
Rust stable	50s
Java 17 (windows-latest)	3m 8s
Rust beta	51s
Java 17 (macos-latest)	1m 14s
Rust nightly	53s
Java 21 (ubuntu-24.04)	1m 43s
Java 21 (windows-latest)	4m 41s
Java 21 (macos-latest)	1m 51s
	54m 51s

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