[9.16] Fix scan-build issue: initialized value never read

Merged Matthijs Mekking requested to merge 3965-source-never-read-master.c-v9_16 into bind-9.16

Backport of MR !7725 (merged)

Make RRSIG queries where the existing tests trigger a DNS_EVENT_TRYSTALE event.

(cherry picked from commit add40273)

Closes #3965 (closed)

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