lalr1.cc: destroy $$ when YYERROR is called.

* data/lalr1.cc (yyreduce): Compute the resulting state before
	running the user action so that yylhs is a valid symbol.
	(yyerrorlab): Since yylhs is complete (it knows its type), we can
	simply call yy_destroy_ to destroy $$ on YYERROR invocations.
	* tests/c++.at (AT_CHECK_VARIANTS): Test YYERROR with variants.

tests/c++.at

@@ -105,11 +105,12 @@ result:
 list:
   /* nothing */ { /* Generates an empty string list */ }
 | list item     { std::swap($][$,$][1); $$.push_back($][2); }
+| list error    { std::swap($][$,$][1); }
 ;
 
 item:
   TEXT          { std::swap($][$,$][1); }
-| NUMBER        { $][$ = string_cast($][1); }
+| NUMBER        { if ($][1 == 3) YYERROR; else $][$ = string_cast($][1); }
 ;
 %%
 
@@ -188,7 +189,6 @@ AT_CHECK([./list], 0,
 [0
 1
 2
-3
 4
 ])