dsa-check-soas: fix error when 0 (or more than 1) records returned

3956f21a moved some processing into resolve_ns. When an unexpected
number of records is returned, an attempt is made to update the
list of warnings, which is not within resolve_ns()'s local scope. The
result is an "undefined local variable" error.

Resolve this by explicitly marking the "warnings" list as in instance
scope. (Although it is not currently a problem, the "OKs" list is also
similarly marked, for future-proofing.)

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>