The reason of the bug is in that `get_var_with_binlog' performs missed
assingment of
the variables as side-effect. Doing that it eventually calls
`free_underlaid_joins' to pass as an argument `thd->lex->select_lex' of the lex
which belongs to the user query, not 
to one which is emulated i.e SET @var1:=NULL.


`get_var_with_binlog' is refined to supply a temporary lex to sql_set_variables's stack.


mysql-test/r/rpl_user_variables.result:
  results changed
mysql-test/t/rpl_user_variables.test:
  a problematic query to be binlogged is added
sql/item_func.cc:
  BUG#19136: Crashing log-bin and uninitialized user variables
  
  The reason of the bug is in that how `get_var_with_binlog' performs missed
  assingment of the variables: `free_underlaid_joins' gets as an argument `thd->lex->select_lex'
  which belongs to the user query, not to one which is emulated i.e SET @var1:=NULL.
  
  `get_var_with_binlog' is refined to supply a temporary lex to sql_set_variables's stack.