The implementation of the method Item_func_reverse::val_str
for the REVERSE function modified the argument of the function.
This led to wrong results for expressions that contained
REVERSE(ref) if ref occurred somewhere else in the expressions.


mysql-test/r/func_str.result:
  Added a test case for bug #18243.
mysql-test/t/func_str.test:
  Added a test case for bug #18243.
sql/item_strfunc.cc:
  Fixed bug #18243.
  The implementation of the method Item_func_reverse::val_str
  for the REVERSE function modified the argument of the function.
  This led to wrong results for expressions that contained
  REVERSE(ref) if ref occurred somewhere else in the expressions.
  
  The implementation of Item_func_reverse::val_str has been changed
  to make the argument intact.
sql/item_strfunc.h:
  Fixed bug #18243.
  Added tmp_value to the Item_func_reverse class to store
  the result of the function. It erroneously replaced the 
  argument before this fix.