httpclient: Fix warning logged by sync HTTPClient destructor

httpclient: Fix warning logged by sync HTTPClient destructor

If an HTTPClient is closed from its destructor instead of an explicit
close() call, it sometimes logs an "inconsistent AsyncHTTPClient
cache" error because the weakrefs appear to get cleaned up in an
unexpected order. Relax the checks in AsyncHTTPClient.close to allow
for a missing value in the instance cache.

Fixes #2539