krb5: Check for signed overflow

krb5: Check for signed overflow

This avoids a compiler error:

../../third_party/heimdal/lib/krb5/krbhst.c: In function ‘srv_find_realm.constprop’:
../../third_party/heimdal/lib/krb5/krbhst.c:113:8: error: assuming signed overflow does not occur when simplifying conditional to constant [-Werror=strict-overflow]
     if (num_srv == 0) {
        ^

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>