fedora: Hits -Werror=format-overflow on s390x

[ingvarha]

This is "gcc (GCC) 10.0.1 20200130 (Red Hat 10.0.1-0.7)", fedora rawhide on s390x:

In function 'varnish_ask_cli',
inlined from 'varnish_launch' at vtc_varnish.c:500:6:
vtc_varnish.c:121:3: error: '%s' directive argument is null [-Werror=format-overflow=]
121 | vtc_fatal(v->vl, "CLI failed (%s) = %d %u %s",
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
122 | cmd, i, retval, r);
| ~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

[ingvarha]

This is varnish-6.3.2