Fixed the same issue with MySQL support as was fixed for pgSQL and SQLite in Quassel commit
[`1fc8eb59a87c005ddfe7d21bc225bef8692b9743`](https://github.com/quassel/quassel/commit/1fc8eb59a87c005ddfe7d21bc225bef8692b9743) -
namely the following:

> ## Make sure that clients can't access buffers belonging to other users ##
> A manipulated, but properly authenticated client was able to retrieve
> the backlog of other users on the same core in some cases by providing
> an appropriate BufferID to the storage engine. Note that proper
> authentication was still required, so exploiting this requires
> malicious users on your core. This commit fixes this issue by ensuring
> that foreign BufferIDs are off-limits.