bug：static int merge_free_chunk(MemoryPool* mp, _MP_Memory* mm, _MP_Chunk* c)

[Zhichao-Yan]

static int merge_free_chunk(MemoryPool* mp, _MP_Memory* mm, _MP_Chunk* c) {
    _MP_Chunk *p0 = c, *p1 = c;
    while (p0->is_free) {
        p1 = p0;
        // bug：向前面搜索邻接内存块貌似有错？p0是作为内存块_MP_Chunk首地址，而不是分配给用户的内存空间的首地址，因此不应该减去MP_CHUNKHEADER
        // ++if ((char*) p0  - MP_CHUNKHEADER <= mm->start) break;
        // --if ((char*) p0 - MP_CHUNKEND - MP_CHUNKHEADER <= mm->start) break;
        p0 = *(_MP_Chunk**) ((char*) p0 - MP_CHUNKEND);
    }

    p0 = (_MP_Chunk*) ((char*) p1 + p1->alloc_mem);
    while ((char*) p0 < mm->start + mp->mempool_size && p0->is_free) {
        MP_DLINKLIST_DEL(mm->free_list, p0);
        p1->alloc_mem += p0->alloc_mem;
        p0 = (_MP_Chunk*) ((char*) p0 + p0->alloc_mem);
    }

    *(_MP_Chunk**) ((char*) p1 + p1->alloc_mem - MP_CHUNKEND) = p1;
#ifdef _Z_MEMORYPOOL_THREAD_
    MP_UNLOCK(mp);
#endif
    return 0;
}

[userpro]

只是确保前面是个合法的chunk（chunk header、chunk end都存在），不过确实可有可无肯定都是合法的chunk，先留着了