Selecting of the CONCAT_WS(...<PS parameter>...) result into
a user variable may return wrong data.

Item_func_concat_ws::val_str contains a number of memory
allocation-saving optimization tricks. After the fix
for bug 46815 the control flow has been changed to a
branch that is commented as "This is quite uncommon!":
one of places where we are trying to concatenate
strings inplace. However, that "uncommon" place
didn't care about PS parameters, that have another
trick in Item_sp_variable::val_str(): they use the
intermediate Item_sp_variable::str_value field,
where they may store a reference to an external
argument's buffer.

The Item_func_concat_ws::val_str function has been
modified to take into account val_str functions
(such as Item_sp_variable::val_str) that return a
pointer to an internal Item member variable that
may reference to a buffer provided.


mysql-test/r/func_concat.result:
  Added test case for bug #50096.
mysql-test/t/func_concat.test:
  Added test case for bug #50096.
sql/item_strfunc.cc:
  Bug #50096: CONCAT_WS inside procedure returning wrong data
  
  The Item_func_concat_ws::val_str function has been
  modified to take into account val_str functions
  (such as Item_sp_variable::val_str) that return a
  pointer to an internal Item member variable that
  may reference to a buffer provided.