MDEV-33769: Memory leak found in the test main.rownum run with --ps-protocol against a server built with the option -DWITH_PROTECT_STATEMENT_MEMROOT

A memory leak happens on the second execution of a query that run in PS mode
and uses the function ROWNUM().

A memory leak took place on allocation of an instance of the class Item_int
for storing a limit value that is performed at the function set_limit_for_unit
indirectly called from JOIN::optimize_inner. Typical trace to the place where
the memory leak occurred is below:
 JOIN::optimize_inner
  optimize_rownum
   process_direct_rownum_comparison
    set_limit_for_unit
     new (thd->mem_root) Item_int(thd, lim, MAX_BIGINT_WIDTH);

To fix this memory leak, calling of the function optimize_rownum()
has to be performed only once on first execution and never called
after that. To control it, the new data member
  first_rownum_optimization
added into the structure st_select_lex.