The error for assignment to __debug__ used ste->ste_opt_lineno instead
of n->n_lineno.  The latter was at best incorrect; often the slot was
uninitialized.  Two fixes here: Use the correct lineno for the error.
Initialize ste_opt_lineno in PySymtable_New(); while there are no
current cases where it is referenced unless it has already been
assigned to, there is no harm in initializing it.