hhvm

The Squangle connection pointer wrapped by AsyncMysqlConnection may be nullptr if the connection was closed or is currently busy waiting for the result of an async query. Most code paths already call either verifyValidConnection() to raise an appropriate Hack exception or explicitly check for and handle a null backing connection, but Query::toString__FOR_DEBUGGING_ONLY() and the SSL-related getters from D33663743 do not, which can lead to segfaults. Slightly simplified reproducer from #8678: and for : Call verifyValidConnection() in all these cases, as raising an appropriate exception (e.g. closed/busy connection) seems appropriate here. Fixes #8678