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/yacc.c

@@ -1641,8 +1641,9 @@ yyreduce:
   yyval = yyvsp[1-yylen];
 
 ]b4_locations_if(
-[[  /* Default location.  */
-  YYLLOC_DEFAULT (yyloc, (yylsp - yylen), yylen);]])[
+[[  /* Default location. */
+  YYLLOC_DEFAULT (yyloc, (yylsp - yylen), yylen);
+  yyerror_range[1] = yyloc;]])[
   YY_REDUCE_PRINT (yyn);]b4_lac_if([[
   {
     int yychar_backup = yychar;
@@ -1782,8 +1783,7 @@ yyerrorlab:
   if (/*CONSTCOND*/ 0)
      goto yyerrorlab;
 
-]b4_locations_if([[  yyerror_range[1] = yylsp[1-yylen];
-]])[  /* Do not reclaim the symbols of the rule whose action triggered
+  /* Do not reclaim the symbols of the rule whose action triggered
      this YYERROR.  */
   YYPOPSTACK (yylen);
   yylen = 0;