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().