In impure push mode, don't allow more than one yypstate to be allocated

since multiple impure parsers would corrupt yynerrs.
* data/push.c (yypstate_allocated): New static global variable
initialized to 0.
(yypull_parse): If yypstate_new returns 0, don't report it as memory
exhaustion if yypstate_allocated is 1, but still return 2.
(yypstate_new): Invoke yyerror and return 0 if yypstate_allocated is
already 1.  Otherwise, set it to 1.
(yypstate_delete): Set it to 0.
* tests/push.at (Push Parsing: Multiple impure instances): New test
case.

src/parse-gram.h

@@ -164,7 +164,7 @@
 typedef union YYSTYPE
 {
 
-/* Line 22 of yacc.c  */
+/* Line 1535 of yacc.c  */
 #line 94 "parse-gram.y"
 
   symbol *symbol;
@@ -178,7 +178,7 @@ typedef union YYSTYPE
 
 
 
-/* Line 22 of yacc.c  */
+/* Line 1535 of yacc.c  */
 #line 183 "parse-gram.h"
 } YYSTYPE;
 # define YYSTYPE_IS_TRIVIAL 1