[v12.x] src: move CHECK in AddIsolateFinishedCallback
CHECK(it->second)
asserts that we have PerIsolatePlatformData
in the per_isolate_
map, and not just a key with empty value. When
it == per_isolate_.end()
, however, it means that we don't have the
isolate and the CHECK(it->second)
is guaranteed to fail then!