authpf: do not sprintf to a null pointer

[emaste]

Summary

The fgetln loop will terminate with buf = NULL at EOF.

Sponsored by:   The FreeBSD Foundation

[emaste]

/tmp/cirrus-ci-build/contrib/pf/authpf/authpf.c:570:17: warning: argument 1 null where non-null expected [-Wnonnull]
  570 |                 sprintf(buf, "%s", "\n\nSorry, you are not allowed to use this facility!\n");
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /tmp/cirrus-ci-build/sys/sys/nv.h:44,
                 from /tmp/cirrus-ci-build/sys/net/pfvar.h:44,
                 from /tmp/cirrus-ci-build/contrib/pf/authpf/authpf.c:31:
/usr/obj/tmp/cirrus-ci-build/amd64.amd64/tmp/usr/include/stdio.h:286:10: note: in a call to function 'sprintf' declared 'nonnull'
  286 | int      sprintf(char * __restrict, const char * __restrict, ...);
      |          ^~~~~~~

[jhb]

contrib/pf/authpf/authpf.c

569

Reusing the buf here was horrible on several reasons.

569

Why not just printf()?

[emaste]

contrib/pf/authpf/authpf.c

569

Attempting to minimize differences from OpenBSD

[jhb]

contrib/pf/authpf/authpf.c

569

Oh that's fair.