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.

data/lalr1.cc

@@ -848,6 +848,7 @@ b4_dollar_popdef])[]dnl
       {
         slice<stack_symbol_type, stack_type> slice (yystack_, yylen);
         YYLLOC_DEFAULT (yylhs.location, slice, yylen);
+        yyerror_range[1].location = yylhs.location;
       }]])[
 
       // Perform the reduction.
@@ -918,8 +919,7 @@ b4_dollar_popdef])[]dnl
        YYERROR and the label yyerrorlab therefore never appears in user
        code.  */
     if (false)
-      goto yyerrorlab;]b4_locations_if([[
-    yyerror_range[1].location = yystack_[yylen - 1].location;]])[
+      goto yyerrorlab;
     /* Do not reclaim the symbols of the rule whose action triggered
        this YYERROR.  */
     yypop_ (yylen);