SQLCancel is possible to be run on already dropped statement

If Stmt handle use is not properly synchronized by the application(and I actually think that the need of this maybe not obvious), it can happen that until SQLCancel actually starts execution when the main thread not only finished the execution SQLCancel should abourt, but even freed. In this case the lock introduced in ODBC-464 will be not of big help if main thread(dropping the Stmt) takes it first. Cancelling thread will crash already at the attempt to read it.
The solution would be introducing of the additional global lock and the global list of deleted statement handles. It looks like a bit too complex as for the problem with such a low probability of happening, but unlikely there is simpler one other than to rely on application