Fix: mmap MAP_FAILED not handled correctly on POSIX

[DavIvek]

Problem

On POSIX, mmap returns MAP_FAILED ((void*)-1) on failure, not NULL: https://man7.org/linux/man-pages/man2/mmap.2.html
Two call sites were incorrectly treating the return value as nullable:

• index_plugins.hpp: allocate() cast mmap result directly to byte_t* without checking MAP_FAILED, returning a garbage pointer instead of nullptr
• bench.cpp: persisted_matrix_gt checked == nullptr instead of == MAP_FAILED,
  silently passing on failure and proceeding with an invalid pointer

Fix

Both are fixed to check MAP_FAILED before casting.