lalr1, yacc: use the default location as initial error location

lalr1, yacc: use the default location as initial error location

Currently lalr1.cc makes an out-of-bound access when trying to read @1
in rules with an empty rhs (i.e., when there is no @1) that raises an
error (YYERROR).

glr.c already gracefully handles this by using @$ as initial location
for the errors.  Let's do that in yacc.c and lalr1.cc.

* data/lalr1.cc, data/yacc.c: Use @$ to initialize the error location.
* tests/actions.at: Check that case.