lalr1.cc: check (and fix) %printer exception safety

lalr1.cc: check (and fix) %printer exception safety

* tests/c++.at (Exception safety): Let the parser support the --debug
option.
On 'p', throw an exception from the %printer.
* data/lalr1.cc (yyparse): Do not display the values we discard, as it
uses %printer, which might have thrown the exception.