yacc: fix push parser

When a pstate is used for multiple successive parses, some state may
leak from one run into the following one.  That was introduced in
330552ea49 "yacc.c: push: don't clear
the parser state when accepting/rejecting".

Reported by Ryan <dev@splintermail.com>
https://lists.gnu.org/r/bug-bison/2021-03/msg00000.html

* data/skeletons/yacc.c (yypush_parse): We reusing a pstate from a
previous run, do behave as if it were the first run.
* tests/push.at (Pstate reuse): Check this.

data/skeletons/yacc.c

@@ -1573,14 +1573,16 @@ yyparse (]m4_ifset([b4_parse_param], [b4_formals(b4_parse_param)], [void])[)]])[
 
   switch (yyps->yynew)
     {
-    case 2:
-      yypstate_clear (yyps);
-      goto case_0;
-
-    case_0:
     case 0:
       yyn = yypact[yystate];
       goto yyread_pushed_token;
+
+    case 2:
+      yypstate_clear (yyps);
+      break;
+
+    default:
+      break;
     }]])[
 
   YYDPRINTF ((stderr, "Starting parse\n"));