Problem: calling non-constant argument's val_xxx() methods 
in the ::fix_length_and_dec() is inadmissible.

Fix: call the method only for constant arguments.


mysql-test/r/ps.result:
  Fix for bug #32137: prepared statement crash with str_to_date in update clause
    - test result.
mysql-test/t/ps.test:
  Fix for bug #32137: prepared statement crash with str_to_date in update clause
    - test case.
sql/item_timefunc.cc:
  Fix for bug #32137: prepared statement crash with str_to_date in update clause
    - call argument's val_str() only for constant items in the 
      Item_func_str_to_date::fix_length_and_dec().