json: enforce a maximum nesting depth for json variants

json: enforce a maximum nesting depth for json variants

Simply as a safety precaution so that json objects we read are not
arbitrary amounts deep, so that code that processes json objects
recursively can't be easily exploited (by hitting stack limits).

Follow-up for oss-fuzz#10908

(Nice is that we can accomodate for this counter without increasing the
size of the JsonVariant object.)