c++: variants: don't leak the lookahead in error recovery

During error recovery, when discarding the lookeahead, we don't
destroy it, which is caught by parse.assert assertions.

Reported by Antonio Silva Correia.
With an analysis and suggested patch from Michel d'Hooge.
<http://savannah.gnu.org/support/?108481>

* tests/c++.at (Variants): Strengthen the test to try syntax errors
with discarded lookahead.

data/lalr1.cc

@@ -901,6 +901,7 @@ b4_dollar_popdef])[]dnl
         else if (!yyempty)
           {
             yy_destroy_ ("Error: discarding", yyla);
+            yyla.clear ();
             yyempty = true;
           }
       }