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.

(cherry picked from commit 098639dc)
79 jobs for !5168 with michal/fix-the-variable-checked-by-a-post-load-assertion-v9_16 in 20 seconds (queued for 16 seconds)
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