Fix the variable checked by a post-load assertion

Instead of checking the value of the variable modified two lines earlier (the number of SOA records present at the apex of the old version of the zone), one of the RUNTIME_CHECK() assertions in zone_postload() checks the number of SOA records present at the apex of the new version of the zone, which is already checked before. Fix the assertion by making it check the correct variable.

This was found while working on #2467 (closed).