test/regress_dns: fix -Wmaybe-uninitialized

test/regress_dns: fix -Wmaybe-uninitialized

Was not noticed since it is reported only with optimization enabled, as
stated in gcc(1):

   The effectiveness of some warnings depends on optimizations also
   being enabled. For example -Wsuggest-final-types is more effective with
   link-time optimization and -Wmaybe-uninitialized does not warn at all
   unless optimization is enabled.

And interesting thing is that it is reported only for -O2, not for -O3,
that's why I did not catched it in both cmake env that I had:
- debug (it has -O0)
- release (it has -O3)

While autoconf has -O2.