Fix warnings in benchmarks

Fix warnings in benchmarks

1. Initialize len in benchmark_compare256.cc.

    In function ‘typename std::enable_if<(std::is_trivially_copyable<_Tp>::value && (sizeof (Tp) <= sizeof (Tp*)))>::type benchmark::DoNotOptimize(Tp&) [with Tp = unsigned int]’,
        inlined from ‘void compare256::Bench(benchmark::State&, compare256_func)’ at /zlib-ng/test/benchmarks/benchmark_compare256.cc:44:33,
        inlined from ‘virtual void compare256_c_Benchmark::BenchmarkCase(benchmark::State&)’ at /zlib-ng/test/benchmarks/benchmark_compare256.cc:62:1:
    /zlib-ng/_deps/benchmark-src/include/benchmark/benchmark.h:480:3: warning: ‘len’ may be used uninitialized [-Wmaybe-uninitialized]
      480 |   asm volatile("" : "+m,r"(value) : : "memory");
          |   ^~~
    /zlib-ng/test/benchmarks/benchmark_compare256.cc: In member function ‘virtual void compare256_c_Benchmark::BenchmarkCase(benchmark::State&)’:
    /zlib-ng/test/benchmarks/benchmark_compare256.cc:36:18: note: ‘len’ was declared here
       36 |         uint32_t len;
          |                  ^~~

2. Make the loop counter unsigned in benchmark_slidehash.cc.

    /zlib-ng/test/benchmarks/benchmark_slidehash.cc: In member function ‘virtual void slide_hash::SetUp(const benchmark::State&)’:
    /zlib-ng/test/benchmarks/benchmark_slidehash.cc:29:31: warning: comparison of integer expressions of different signedness: ‘int32_t’ {aka ‘int’} and ‘unsigned int’ [-Wsign-compare]
       29 |         for (int32_t i = 0; i < HASH_SIZE; i++) {