Avoid -Werror=strict-overflow on in rk_dns_srv_order()

Avoid -Werror=strict-overflow on in rk_dns_srv_order()

In a strict Samba build with -Werror=strict-overflow on Ubuntu 18.04
with gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)
we see

../../source4/heimdal/lib/roken/resolve.c: In function ‘rk_dns_srv_order’:
../../source4/heimdal/lib/roken/resolve.c:639:7: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
     if(num_srv == 0)
       ^
cc1: all warnings being treated as errors

This avoids the issue by additionally setting a distinct flag.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>