Avoid format-truncation warning
Avoid format-truncation warning

The existing code gives this error when compiled with GCC 12:

../sway/server.c: In function ‘server_init’:
../sway/server.c:217:75: error: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size 8 [-Werror=format-truncation=]
  217 |                 snprintf(name_candidate, sizeof(name_candidate), "wayland-%d", i);
      |                                                                           ^~
../sway/server.c:217:66: note: directive argument in the range [-2147483647, 32]
  217 |                 snprintf(name_candidate, sizeof(name_candidate), "wayland-%d", i);
      |                                                                  ^~~~~~~~~~~~
../sway/server.c:217:17: note: ‘snprintf’ output between 10 and 20 bytes into a destination of size 16
  217 |                 snprintf(name_candidate, sizeof(name_candidate), "wayland-%d", i);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Because i is never negative, this is a false positive, but it is easy to
change i to unsigned to silence the error.

1ace/sway,1ace/sway,1ace/sway