Fortified string checks report out-of-bounds write

[Rich FitzJohn]

In function ‘memcpy’,
    inlined from ‘mdb_node_add’ at lmdb/mdb.c:8184:4:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34:10: warning: writing 8 bytes into a region of size 1 [-Wstringop-overflow=]
   34 |   return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘memcpy’,
    inlined from ‘mdb_node_add’ at lmdb/mdb.c:8178:5:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34:10: warning: writing 8 bytes into a region of size 1 [-Wstringop-overflow=]
   34 |   return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Requires very new gcc-10