sockets: avoid string truncation warnings when copying UNIX path

sockets: avoid string truncation warnings when copying UNIX path

In file included from /usr/include/string.h:494,
                 from include/qemu/osdep.h:101,
                 from util/qemu-sockets.c:18:
In function ‘strncpy’,
    inlined from ‘unix_connect_saddr.isra.0’ at util/qemu-sockets.c:925:5:
/usr/include/bits/string_fortified.h:106:10: warning: ‘__builtin_strncpy’ specified bound 108 equals destination size [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘strncpy’,
    inlined from ‘unix_listen_saddr.isra.0’ at util/qemu-sockets.c:880:5:
/usr/include/bits/string_fortified.h:106:10: warning: ‘__builtin_strncpy’ specified bound 108 equals destination size [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We are already validating the UNIX socket path length earlier in
the functions. If we save this string length when we first check
it, then we can simply use memcpy instead of strcpy later, avoiding
the gcc truncation warnings.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190501145052.12579-1-berrange@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>