At logging a first Query referring a user var, the slave missed to log the user var.
It appears that at execution of a Uservar event the slaver applier
thought of the variable as already logged.
The reason of misjudgement is in coincidence of query id:s: of one that the thread
holds at Uservar execution and another one that the thread sees at the Query applying.
While the two are naturally different in the regular execution branch (as two computational
events are separated as individual events), in the deferred applying case the User var execution
effectively belongs to its Query processing.

Fixed with storing the Uservar parsing time (where desicion to defer is taken) query id 
to temporarily substitute with it the actual query id at the Uservar execution time
(along with its query).
Such manipulation mimics behaviour of the regular applying branch.

sql/log_event.cc:
  Storing the Uservar parsing time query id into a new member of the event
  to to temporarily substitute
  with it the actual thread id at the Uservar execution time.
sql/log_event.h:
  Storage for keeping query-id in User-var intance is added.