managed_bytes: Fix compilation on gcc 8.2
managed_bytes: Fix compilation on gcc 8.2

The compilation fails on -Warray-bounds, even though the branch is never taken:

    inlined from ‘managed_bytes::managed_bytes(bytes_view)’ at ./utils/managed_bytes.hh:195:22,
    inlined from ‘managed_bytes::managed_bytes(const bytes&)’ at ./utils/managed_bytes.hh:162:77,
    inlined from ‘dht::token dht::bytes_to_token(bytes)’ at dht/random_partitioner.cc:68:57,
    inlined from ‘dht::token dht::random_partitioner::get_token(bytes)’ at dht/random_partitioner.cc:85:39:
/usr/include/c++/8/bits/stl_algobase.h:368:23: error: ‘void* __builtin_memmove(void*, const void*, long unsigned int)’ offset 16 from the object at ‘<anonymous>’ is out of the bounds of referenced subobject ‘managed_bytes::small_blob::data’ with type ‘signed char [15]’ at offset 0 [-Werror=array-bounds]
      __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
      ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Work around by disabling the diagnostic locally.
Message-Id: <48942ddec60a2d40c39d79b3577296db99c713a8@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla