underflow in stats channel stale cached RRSIG gauge [ISC-support #14769]

A customer running a Supported Preview build has started seeing:

<rrset><name>#NS</name>
<counter>18446744073709551615</counter></rrset>

<rrset><name>#RRSIG</name>
<counter>18446744073709551615</counter></rrset>
</cache>

The <rrsig> elements are part of the block that measures the current number of records in the cache by RRTYPE, and the # prefix on the RRTYPE name indicates that these are "stale" records. By the version reported these are supposed to track the number of records that could be used by the serve-stale feature (if enabled).

An underflow on the stat seems the most likely explanation.

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